I've used Snapfish once. Good quality prints and as it was my first order very cheap (free prints first time you use). You also get an online album and loads of other stuff available like printing on T-shirts and mugs.
Haven't used since simply because I rarely want prints.
I use photobox (think either twiglett or nutcracker recommended them once a million years ago). I've never had a problem with them and the prints arrive really quickly. I also get them sent overseas (to SA) and not once has the package gone missing (and that's really saying something!).
I've noticed that ofoto do them as well (v useful if you have a Kodak camera) but I've never tried them.
I always use Truprint - cheap if you pre-buy prints off them and excellent service, always v quick delivery (to me anyway!)
I'd pre-bought 250 prints and a few weeks later they reduced their prices, so gave me more prints to compensate - I liked that!
Another vote for Photobox here, I did a leaving card for a work colleague (got all his work colleagues to hold up letters to form words and then made a collage) and the photos came next day. Spent a few days watching out for his lunchbreak then assembling the troops!
He was really pleased the result and Photobox were really reliable - they also up your memory capacity when you place an order.
Yet another vote for photobox - I've really found them to be excellent - have used them for a wide range of stuff, including calendars as well as various different sized prints and the quality is consistent and reasonably priced.