Actually, you asked "what do you guys think about a single 18" and you got a whole bunch of answers that described what people thought!
Its hard not to get into the science when talking amps and cabs, because it is a science-based topic, so you're going to bring out the minions right away with a wide entry point like your original post.
Where I was going was in line with others. For your sonic needs, many things need to be considered, but speaker size itself is not terribly important, unless your sonic needs are also influenced by your emotional desire to have something big.

My Acme B2 puts out more bone-shaking lows than my 1x18 sub does, and it only has two 10's providing the bass. Size has little to do with it.
The point is, if you want your 18's then go for it, but you won't necessarily be getting anything lower than you would if you had 410s instead - it really depends on what speakers are used and how the cab is designed.
My real world experience with them: I had two peavey 1x18s in a stack for a while and they are low, but a bit muddy in comparison to the identical 4x10. They both go low, but the 10's were "snappier" (I hate descriptors like that!).
Currently I have a Behringer 18" PA sub that I play either with a 2x10 or a 1x15. My 1x15 sucks so its like a natural low pass filter anyway, and combined with the sub is a perfect wall of thud for dub. But really, we're talking a lot about perception here - if you hear something and it sounds good to you, that's all that really matters...