I generally use Costco for my 35mm C-41 processing, I wish that they had the capability to do 120/220, but there isn't a big enough market I guess....
wouldn't be that hard to train their people to do it... but its not my decision. While at school right now, I'm processing everything myself in the school's darkroom. I have unlimited access to color chemistry, and I'm making sure I take full advantage of it .
Otherwise, I've found that shooting a 99 cents store roll of film and taking it to my local labs that have a decent turnover lets me know who takes care of their equipment (mainly to see if there's roller transport marks or scratches on the negatives, etc)
so far, one of my local costco's (azusa, CA for anyone close to that store) doesn't do the cleanest processing IMO, but the San Dimas store does super clean processing and printing, so it can vary, even within a company such as costco