I use ms. pinky http://www.mspinky.com/ timecoded records along with a shitty esi u46dj soundcard.
Two things:
1) there is a definitely loss in quality when doing software 'pass-thru' of non-timecode records, in my case this is down to the shit-ness of the soundcard. Instead of doing software pass-thru, i have a phono splitter that connects both turntable and soundcard to mixer simultaneously, so can just use the mixers own "phono / line" to switch between vinyl / mp3.
2) despite what people say, these mp3 vinyl rigs are not easy to set-up. Sure, in your bedroom or studio it's fine, but when you take it to a club it can be a nightmare. If it turns out that the club setup is shit, for example, only the left turntable channels works (most timecode systems use channel seperation so need both left+right), or the needles are bogey, or no place to put laptop + soundcard, etc.
I use it only in my room or mate's house, vinyl is so much more reliable. altho the last gig i had, the guy playing after me was using a laptop, and had it setup, so even when i was using vinyl i still had people saying "this is shit, he's only playing off a laptop!!" cocks. ;(