I've used moo.com for my business cards for ages (I get the half-size "minicards"), so I knew immediately I wanted to use them for our invitations. DS & I chose a single image of us and had them print two 20-card packs of it for our invitations, with all the info on the back. The photo sides of the cards were glossy, which surprised me a little since the minicards are always printed matte, which I prefer, but I know a lot of people like glossy. They still look great, and the cardstock is VERY high quality and weight--heavier than a lot of postcards you'd buy at an airport gift shop, that's for sure.
The cards are $20 for a 20-pack or $50 for a 60-pack. They also sell packs of matching envelopes for $7-$10 per 20-pack; we got the metallic gold ones for $10/pack and provided our own white outer envelopes to avoid problems with hand-canceling. The postcards are not quite a standard size, so you can put them in A6 envelopes but there will be a bit of wiggle room; if you want a perfect, tight fit, you should order from moo. With our announcements, we ordered two 100-packs of red envelopes from envelopemall.com for a total of $26.22 with shipping, so that was definitely a significant saving, and they traveled fine in those envelopes.
With the announcements, we actually got 12 different photos in 3 packs of 60, so we had 15 of each photo (5 per pack). It's no extra charge to get multiple photos (the text will be the same on all of them); if you wanted to get 60 different photos in a 60-pack, you could. It was fun having a selection of photos to send to people, allowing us to better tailor announcements to specific friends and family.
My only complaint is that I feel like the shipping costs are a bit unreasonably high, especially since they often end up arriving a day or two later than the site tracking info says they will. If you give yourself some extra time, you could save on this by waiting until they have one of their free shipping offers, which happen every couple of months or so.
If you like custom photo cards, these are VERY high quality. Even if you prefer a more traditional invite, definitely consider them for fun, personalized announcements!