I've been using
a Boss RC-20 for a few years. Generally I'm happy with it, but some of its limitations make it difficult to work with:
1) After laying down the first loop, it takes a
split second to calculate the tempo. This means in a live situation, you can't record the loop then have it play right away. You have to start recording, stop recording, then start playing it. (Theoretically you can set the tempo first, then start recording, but good luck doing this live in the middle of a song. Useless.)
2) There's a metronome, but this feature is pretty much useless except in rehearsing situations. If you could output the click just to headphones, it would be useful live. But who wants to hear "TAK! BANG! BANG! BANG!" coming out of their amp? Not my audience.
3) There's only one level of undo. Not a bad limitation for the price, but still.
4) An option to fade the loop out rather than just stopping it would be nice.
Overall, the RC-20 is great for rehearsing and idea generation, but for live use it's always a struggle.
Price-wise, the next step up would be the RC-50, but I've read a lot of bad reviews about it.