I personally started out with a 55 gallon and have continuously "down graded" to smaller tanks. I personally find that the things I enjoy in a reef are the really tiny things and these can only be seen well in a small tank. When I look at these huge 300 gallon reefs, I'm amazed at the beauty and complexity of them; but can't focus on the small life that is sure to teeming in such a large system.
My last tank was a 10 gallon and I've decided that this was too small for my needs. At this point I'm working on a 25ish system (build thread coming once I find the aquarium I want at the price I want). I've found that, for me, its a good happy medium for stability vs. seeing the small life. At some point I'll probably work on a pico to satisfy my DIY bug, but I doubt it will be a long term system. Once I'm satisfied with it, I'll probably return its inhabitants to my larger system. Who knows what the future holds.