I used them for my STD postcards...but that was before I knew of Vistaprint. They quality between the two is comparable, but with VP you can get 100 free, so it'd much cheaper.
I ordered invitations and reply cards and address labels from them. I really like the feel of the paper. It reminded me a lot of photo paper, but a little textured and a little thicker. Based on the price, I loved what I got and it only took 3 days for them to print and get them too me. They are super fast!
I used Vistaprint. I got save the date magnets through them and I love the quality, price and style. I paid $17. for 100. Now I get 25% off cupons for them and will probably use them again for invitations. Not sure about 123 print but if you want to check out vistaprint they will send you a wedding package of samples to give you an idea of what they offer.
Re: Quality of 123print?
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Jessica
123print will send you samples too if you would like to get a better idea of what they offer.