If I care enough, I use whcc. The quality is superb but pricier and have to pay shipping if I don't meet the minimum order.
When I don't care, like if I use them for scrapbooking where embellishments often grab more attention, I use Target or Costco. I use Costco when I want the matte finish or white border. The quality is not bad. I prefer Costco because
I like matte photos. I haven't tried Costco's glossy photos.
I hear raves about smugmug prints. They use Bay Photos which is similar to whcc, a pro lab. If you don't feel like opening an account at whcc, you can consider using smugmug instead. They have an interest test result that covers MPIX. The result is not in MPIX's favor, but it's compared against pro labs so the bad review is expected. What's alarming though, it seems MPIX's
colors are very inconsistent. They sent the same photo to MPIX under different customer names and the returned photos are so drastically differnet it's shocking. It's something you should consider, especially if you want them to color
correct for you.
Correction: I just saw that smugmug uses ezprints. They only recently added Bay Photo, but I don't think you can choose Bay Photo unless you have a pro account.