Although I myself am still waiting for an upgrade to 4GB to get a MBA, my partner has a Rev A HDD MBA and a close friend a Rev B SSD MBA and I've used both machines quite a bit.
I think the capabilities of the MBA are more than sufficient for 95% of users, the main exception being of course a lot of photo/video editing and gaming.
But even for some Photoshop now and then and some Aperture and (Rev B) for some casual gaming the MBA is really fine.
- If you are pondering about saving money and getting a Rev A, don't. The MBA has much less heat issues in Rev B and because of the new nvidia chip used in Rev B, you will likely see performance gains in SnowLeopard with OpenCL
- Also, as often said already in the board, the SSD used in RevB is extremely fast and worth every $.
- About Firewire: Well, I've had my first generation MB now for 2.5 years and the only two times I needed Firewire was to transfer videos from my old DV camcorder (the new ones all have USB 2) and to test a satellite TV box from Elgato (the new TV sticks, again, all have USB 2). My external drive can be operated through USB, Firewire 400 and 800.
So if you don't already have a device that uses firewire you won't miss it. Although it is faster when using external drives, for simple time machine backups and storage USB 2 is enough...