Hi there,
Sorry for the late reply - I am not usually on this forum, but saw this post as I was searching through, and wanted to let you know about my experience.
I am a nontraditional student who worked in Seattle for several years before taking my postbac classes at UW. I have been at it for two years and am currently in my final quarter, finishing up biochemistry.
Other threads do a good job of covering the downsides of a UW postbac - for example, this one:
http://forums.studentdoctor.net/showthread.php?t=525481.
My experience has been different. I had an excellent experience doing my postbac at UW and would do it again in a heartbeat. It is true that classes are very large, but I have been really impressed by the quality of instruction across the board. I did my undergrad at a top-ranked university and the instructors I have had at UW are easily on par with my original undergrad instructors, if not better. The intro Biology series at UW, in particular, is very well put together. I also found that most UW professors are very friendly and genuinely interested in helping students learn. I regularly attended office hours, and by the end of the quarter, most of my professors knew me by name (and a couple of them wrote med school recommendation letters for me).
The other thing that is amazing about UW is that it is a huge medical research center with a ton of undergrad-level research opportunities. Undergrad participation in research is not just tolerated, it is actively welcomed. I found an opportunity where I got to know my PI (a professor) well, spent a lot of one-on-one time with him and got what I think was a strong recommendation letter as a result.
UW can feel a bit impersonal because it is so large, but I have gotten to know a number of my classmates who have gone through the whole pre-med sequence with me, and they have been great for study groups and to sit with in class. A couple of these classmates are also older nontraditional students with interesting prior backgrounds.
Enrolling in premed classes as a non-degree student is a bit of a crapshoot, but in my first quarter (spring quarter), I was able to enroll in Bio 180 and Chem 142 successfully (although my enrollment in Chem 142 came right down to the wire). I applied for formal postbac standing twice and got in the second time, so I didn't have any registration problems afterwards. I do know a couple of people who have been able to take all their prereqs successfully through non-degree enrollment, but your experience may differ.
As a postbac student, you get access to UW premed advising. My adviser has been excellent and incredibly supportive, though I get the sense that the premed advising office may be a little understaffed.
I have been applying to med school in the current cycle and have two acceptances so far at top schools, one of which is UWSOM.
So in summary - I had an excellent experience and would not hesitate to do my postbac at UW again, but be aware of the downsides (covered in other threads).