Sounds like fun. I don't know how much writing I would get done in a coffe shop, especially if they served cake. I might be eating more than writing.
For me, it works - especially if I don't take the computer. Fewer distractions than home. I may go to one here in a few minutes - there's one that I've been going to almost every Saturday. It's right across from a grocery store I like, so I can write and have coffee and then run my weekend errands.
Write-ins are great motivation, too...because if there are a dozen other people around you typing away, you feel like you *have* to keep going.