thegreenkangaroo.com or tgkdesigns.com we chose the layout and stuffed them ourselves. it was really easy to "do it yourself" the paper quality was beautiful and you can ask for samples on the ink and paper as well. they arrived on time, no complaints. loved them! i even used their designs for our wedding website. just copy and paste!
We designed our own and got them printed through Vistaprint! They were cute, creative, good quality, and really affordable (go to retailmenot.com or couponcabin.com for some discount codes). If you want to save yourself even more money, have people reply via your wedding website (on theknot) or via phone (for those who aren't internet-savvy) -- this way, you save money on return postage, and your outgoing envelope will be cheaper because you won't have to worry about an extra envelope in there! It takes some extra time in setting up the website response, but once it's up and running, it's TOTALLY worth it!! :)