I just have an RP50... Got it for free and it's pretty sweet. Not the best distortion though my crappy amp (then again the distortion on it plain sucks anyway)... with headphones it sounds pretty good.
Guitar/bass/mandolin stuff:
PRS SE Custom 24
PRS SE Paul Allender
Martin DCX1E
Squire Start
Memphis Bass
Johnson Savannah mando

Digitech RP1000
Line 6 Spider Valve 212
Toneport Studio KB37

check out my last.fm!
I tried the RP55. The effects were good, but it was pretty bad at modeling. There was too much bass that I couldn’t equalize out (the low strings sounded nearly twice as loud at they should have). Distortion is very dirty at lower settings (in the bad pops-and-crackles way) and cranked it sounds silly, like it was based on how a child thinks a distorted guitar sounds. If you already have an amp and just want one to use for cheap effects go for it.

From what I’ve read the effects get much better and have finer settings in the high-end models. Some reviewers also say that the modeling is better, but I’d be wary about that.

Be aware that the low-end model and DC adapter cost as much as the next model up which comes with an adapter.
RP1000 is obviously the best one. Its pretty cool for sure, and I like that you can bypass the amp modeling if you want more of your amps character, or bypass your amps preamp if you just want the amp model sound. That said, I like the Line 6 POD X3/M13 more than anything in the RP series, so I suggest looking into those as well
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