tl;dr: Right in all the right ways, this is one of the two tripod mounts I keep in my bag.
There are finally a plethora of different iPhone 4 tripod mounting solutions on the market, and this is a very, very good thing. You can browse other Reviews on my website to find some examples of other great tripod mounts—some very nice for specific purposes, others that are only marginally better than duct-taping your iPhone to a tripod!
The SnapMount is a case very similar to what I had in mind back when I was thinking about investing in manufacturing a few hundred to sell (but mostly so I could use one for myself). The plastic cover for the iPhone has two embedded metal tripod screw holes; one on the long axis, and one on the short axis, so you can mount the iPhone vertically or horizontally.
The tripod mount also has extended plastic tabs on all four corners of the iPhone that make it extremely easy to remove the iPhone from the mount (a problem I've had with some cases and holders is that it feels like I'm going to break the glass on my iPhone!).
My new on-the-go kit: Snapmount, iPhone 4, Mini tripod (not pictured: Glif).
This mount isn't quite perfect—because of the tabs, it doesn't fit in a small space quite as well as, say, the Glif (which is the other mount in my bag at all times), but it feels a heck of a lot more safe to strap my iPhone into the SnapMount than any of the other mounts I've used so far.
The tripod socket is very firm.
The ports and buttons on the iPhone are very accessible; the volume buttons and silent switch are the only things that are a little more difficult to use. The headphone jack is also easily accessible.
Build quality = great, shipping time = great, usability = great, stowability = okay.
I'd definitely recommend this if you don't need a mount that is tiny/sitting in the bottom of your bag. If you can store a mount that's just slightly larger (in volume taken in a bag) than the iPhone 4, this is a great option.
Buy it from Amazon for $14.50.