Since both adjustable wattage and adjustable temperature irons have been mentioned, I think it's important to differentiate between the two.
Irons without any control just heat at their rated wattage 100% of the time. This is bad because that wattage may or may not be appropriate for a specific job. And - one thing is certain - when the iron is just sitting still on your desk not being used, it is definitely inappropriate to have it cooking itself at full blast! An iron with no control is equivalent to driving your car with the gas pedal bolted to the floor. As soon as you turn the car on, you are at 100% full throttle, all the time - regardless of if you're trying to carefully pull into a parking spot, going up or down hills, or you're flying down the freeway. If you buy a 30 watt iron, and the types of soldering you're doing need about 30 watts, well - you're okay, when you're actually soldering. But there will be cases where you might wish you had more or less. And regardless of if it's an appropriate wattage for a specific solder job or not, irons can basically damage themselves if left on and at full blast with nowhere for that heat to go - at the very least, you'll oxidize the tip.
Some irons (like the Weller WLC100 which has been mentioned a few times) have a control with arbitrary numbers (i.e. 1 through 4, or 1 through 10). These controls are essentially variable wattage controls. The highest number is full-on where you get the rated wattage 100% of the time. When you select a different number, you get a lower wattage. So, if "4" means 50 watts, "2" might mean 25 watts. This style of iron is equivalent to being able to fix your gas pedal in one of a set number of positions. But if you fix it at the halfway point, you're at halfway no matter what. At least you get a choice - if you're soldering big huge terminals on thick wire, you can turn it all the way up. Or if you're soldering a really small sensitive component without any heatsinking capacity, you can turn it down to prevent damage. But you still just get X wattage, all the time, whether the iron is sitting in the holder or actually touching a part.
Besides the risk of burning your tips up if wattage is too high, there's the frustrating slow recovery of an iron with wattage that's too low (or set too low, if adjustable). So if you take your adjustable iron and set it to an appropriate low setting, in the hopes of keeping your tip and your parts safe, it will take a much longer time for it to warm back up between solder joints. To continue the car analogy, you might be okay moving along on flat ground at half throttle, but every once in a while you come up to a big hill and since your gas pedal is fixed at half throttle, you struggle to make it up the hill.
Meanwhile, an iron with a temperature controller is like a car with cruise control. You no longer have to worry about the gas pedal. You just pick the speed you want to go, and the car moves the gas pedal for you. Set it at 55 and you go 55 regardless of any big long hills. If your choice of solder works well at 320 C, you just set the iron to 320 C and forget about it. When the iron is sitting in the holder unused, the temp controller will pull back on wattage and keep the iron from overheating. But as soon as you touch the tip to a big thermally conductive part, the iron kicks the wattage way up and you are able to maintain temperature despite quickly heating up a lot of metal.
As such, the "ultimate" setup in terms of flexibility is really a high wattage iron (so it has a high capability to move a lot of heat no matter the part) with temperature control (so it stays at the ideal temperature - it doesn't burn itself out, and it doesn't cool down every time you use it). Even if you always set the temperature control to the same exact number, the iron will perform better and last longer than an iron without a temperature controller. The value in the controller is not just being able to pick a specific temperature, but also having an iron that will manage the wattage applied to the tip in order to maintain that temperature over a range of conditions.
Irons without any control just heat at their rated wattage 100% of the time. This is bad because that wattage may or may not be appropriate for a specific job. And - one thing is certain - when the iron is just sitting still on your desk not being used, it is definitely inappropriate to have it cooking itself at full blast! An iron with no control is equivalent to driving your car with the gas pedal bolted to the floor. As soon as you turn the car on, you are at 100% full throttle, all the time - regardless of if you're trying to carefully pull into a parking spot, going up or down hills, or you're flying down the freeway. If you buy a 30 watt iron, and the types of soldering you're doing need about 30 watts, well - you're okay, when you're actually soldering. But there will be cases where you might wish you had more or less. And regardless of if it's an appropriate wattage for a specific solder job or not, irons can basically damage themselves if left on and at full blast with nowhere for that heat to go - at the very least, you'll oxidize the tip.
Some irons (like the Weller WLC100 which has been mentioned a few times) have a control with arbitrary numbers (i.e. 1 through 4, or 1 through 10). These controls are essentially variable wattage controls. The highest number is full-on where you get the rated wattage 100% of the time. When you select a different number, you get a lower wattage. So, if "4" means 50 watts, "2" might mean 25 watts. This style of iron is equivalent to being able to fix your gas pedal in one of a set number of positions. But if you fix it at the halfway point, you're at halfway no matter what. At least you get a choice - if you're soldering big huge terminals on thick wire, you can turn it all the way up. Or if you're soldering a really small sensitive component without any heatsinking capacity, you can turn it down to prevent damage. But you still just get X wattage, all the time, whether the iron is sitting in the holder or actually touching a part.
Besides the risk of burning your tips up if wattage is too high, there's the frustrating slow recovery of an iron with wattage that's too low (or set too low, if adjustable). So if you take your adjustable iron and set it to an appropriate low setting, in the hopes of keeping your tip and your parts safe, it will take a much longer time for it to warm back up between solder joints. To continue the car analogy, you might be okay moving along on flat ground at half throttle, but every once in a while you come up to a big hill and since your gas pedal is fixed at half throttle, you struggle to make it up the hill.
Meanwhile, an iron with a temperature controller is like a car with cruise control. You no longer have to worry about the gas pedal. You just pick the speed you want to go, and the car moves the gas pedal for you. Set it at 55 and you go 55 regardless of any big long hills. If your choice of solder works well at 320 C, you just set the iron to 320 C and forget about it. When the iron is sitting in the holder unused, the temp controller will pull back on wattage and keep the iron from overheating. But as soon as you touch the tip to a big thermally conductive part, the iron kicks the wattage way up and you are able to maintain temperature despite quickly heating up a lot of metal.
As such, the "ultimate" setup in terms of flexibility is really a high wattage iron (so it has a high capability to move a lot of heat no matter the part) with temperature control (so it stays at the ideal temperature - it doesn't burn itself out, and it doesn't cool down every time you use it). Even if you always set the temperature control to the same exact number, the iron will perform better and last longer than an iron without a temperature controller. The value in the controller is not just being able to pick a specific temperature, but also having an iron that will manage the wattage applied to the tip in order to maintain that temperature over a range of conditions.
