Yeah, but what's the catch? Are the videos overly compressed? Is the audio or video quality severely or even slightly compromised? Or do you get a perfect transfer from BR disc to iMac, with full 1080p video and lossless audio, for instance?
I am relatively new to using Handbrake but here are some of my observations:
Handbrake has quite a few built in presets that you can use to rip your movies. Using a high profile preset will get you the largest size file with the "best" picture quality. You can also use presets for

TV and/or ipods. So you have some control as to the quality of the rip.
In my experience, playing a ripped mkv file of an SD movie on my iMac is as good if not better picture quality than playing the movie from the DVD. I believe xbmc and Plex have some sort of upconverting feature built in to achieve this. (I'm not 100% sure on this, hopefully someone else can chime in here)
As for blu ray movies, I have experienced 2 different scenarios. The

TV is only capable of playing back at 720, so you do lose 1080 when playing back through the apple tv. However, a high profile rip played back on the iMac via xbmc or Plex looks awesome on my 27" iMac.
To answer your question, I suppose any time you're doing some conversion to the files there may be a slight compromise with audio and video, but I don't notice it enough to be concerned about it, and in the case of SD DVDs I think picture quality is actually better on the ripped files vs DVD playback.
I really wish Apple would have added Blu Ray to the Mac line but I knew of this issue before I bought mine.... It's a pity because the iMac display is a beauty. But simply following the steps mentioned you can have HD movies playing on your machine in no time