I'm currently testing dimmable CFL's to replace a bunch of R30/PAR30 halogens.
The Global Consumer Products ("Overdrive") Dimmable 5000K 15W/65Wequiv R30 A) sticks out beyond the can, B) advertises only 43 lumens/watt, C) takes a little while to warm up, D) doesn't do a cold start to minimum brightness, E) sometimes cuts out if you dim it too fast, and F) produced a nice, clean white light. 2 year warranty... _not_ Energy Star.
The Neptun 5000K Dimmable 18W/75Wequiv standard A) sticks out beyond the can a little (and is a regular bare twisty CFL), B) advertises 61 lumens/watt (and is indeed visibly brighter), C) comes on immediately D) cold starts immediately to minimum brightness, E) does not cut out, and F) also produced a nice, clean white light. 2 year warranty, and it is Energy Star.
Others to be tested when they arrive, but so far the slightly more expensive ($11.xx vs $9.xx from 1000bulbs.com) Neptun is winning. The Neptun also had less dire warnings on the instructions than the Global did.
Neither one dims nearly as much as an incandescent. The Neptun dims about as far as the Global... despite starting off significantly brighter (almost twice the lumens).