The technical quality is great. I printed about a hundred 4x6es and some bigger ones and they all look really good (well; as good as the photos allow them to be).
By default they print matte; but you can add a "lustre" coating. It's not quite glossy, but gives colorful photos a little extra boost. On a black and white photo where I tried the coating I think it was a mistake. On most of the color photos where I tried it, the lustre version is better.
On a 16x20" print I tried their mounting (double weight matboard) and it's much more impressive than I expected.
Likewise I had some 8x12 photos framed and the frames and beautiful and well done - much much better than their website makes it look. The framing (with non-glare glass) was about $30 which seemed like a lot for a small frame, but it's high quality so the price is about right.
However, the neatest thing is that they package my $3 prints like it's an expensive piece of art. And don't even get me started on how well they packaged the framed photos. You know how it's neat to open an Apple product? Yeah - between the excitement of seeing the photos and how nicely they packaged them, that's about how it's been to open the boxes they sent. Shipping is $7 per order or $11 for overnight shipping. That's somewhere between very reasonable and insanely cheap all things considered.
So, Mpix: Highly recommended so far. I'm looking forward to trying them again.