I've never been a fan of my effect loop, at least for effects uses.
The signal is lower (I think someone said lower impedance) coming through the effects loop, so I can't dial in any usable distortion through my pedals... Also, most of my other pedals just sound different in the loop.
If I did use it in its designed function, I'd put my volume and delay pedals in the loop and leave everything else in front... But between that and the amp's footswitch, I'd have four cables between the amp and the pedalboard. It's just not worth that.
As it is, I sometimes use one effects loop (I have two on my Fender) for a line out, sometimes for a handy tuner output. I used to use the effect loop to take a line out to a bigger slave amp, back before I realized the joy of using PA support.
Now days, the effects loops go largely unused for shows. If my drummer can't hear me, I take a line out straight to his monitor mixer so he can dial me in specifically as much as he wants. I was using the effect loop faithfully for my tuner because I coudn't get it to work for the life of me in my pedal board. Turns out I was using a faulty patch cable, so now it's back on my board.
Occasionally I'll use the IN from my effects loop if I'm doing something that I want to stay uncolored, like playing an acoustic guitar... or, heaven forbid, I have to plug a mixer into my amp...