They have the same cylinder length. They are the same size as their .38 counterparts. I can't tell you whether the j-frame has the std .38 j-frame or the .357 j+-frame (probably the former, given that the 9 j-frames were released before the .357s). Regarding dependability, I've never had a problem but I've heard of problems arising from bent moon clips and protruding +P primers.
I have a S&W 940. It does not require moon clips to shoot (headspaces on the case mouth) but they are needed if you want the cartridges to eject. (Moon clips are cheap and readily available from Brownells and other sources.) I believe that the only DA 9mm revo that did not require moon clips to eject is the S&W 547, which had a pretty neat extractor setup. (This is the model to which glockdoc refers.)
My 940's accuracy is exactly comparable to other snubbies. I have to say, I saw no use for these 9x19 revos when they first came out. However, after years of schlepping .38 snubs and .357 snubs, I finally noticed that the 9x19 appears to be more efficient from this type of revolver. From a 2" (revo) bbl, the velocity for a given load is usually only slightly less than it is from a 4" (semiauto) bbl. (Remember the difference in how revo and semiauto bbls are measured, and you'll note they're almost the same length - but for the bbl/cyl gap.)
Eventually, I got tired of the flash of .357 powder burning outside the snubby bbl, and thought I'd go with a 9mm. Admittedly, the 9 doesn't give you the range of bullet weights a .357 does, but when was the last time you fired a 180-grainer out of a snubnose? (I have, btw, and it wasn't pleasant!) I'm presently debating whether 115 gr +P hps or 147 gr Ranger hps are a better carry load (leaning toward the heavier bullet out of the small pistola), but I'm confident the either one is fine. I really feel the 9x19 gives the best efficiency of the available j-frame cartridges.
Regarding recoil: the 9x19 is (as is to be expected) a little snappier out of the short bbl than is a .38 bullet of similar weight - about like a .357 of similar weight. There is no question in my mind that a heavier-bulleted .357 cartridge out of a snub produces significantly more recoil than does a heavier 9x19 cartridge. And 9x19 blasting ammo is so dang cheap for practice (although no cartridge provides the pleasant shooting experience of a 148 gr .38 wadcutter - all the 9s are moderately snappy to shoot).