I'm now at a couple weeks with this as my only "cell" phone. I say "cell" in quotes because I actually just pulled the $20/month card out of my Mifi device and shoved it into this device (it uses micro-SIM, I believe). (I use a GSM carrier, which kind of narrows it down here in the USA.)
Also good... Geeksphone just released Firefox OS 2.0 for this device. Honestly, I thought 1.3 was decent, so to me 2.0 seems like a much more minor upgrade than the big version number bump would indicate. Concurrent with the 2.0 release, they also gave out all the source code along with build instructions. Not that most mortals will avail themselves, but it's good to know that there are tech wizard eyeballs on the code of your phone!
And even more good: the camera takes pretty decent pictures, at least to my eye. The screen looks pretty good, and the phone manages to play mp3, wav, and ogg files, even while streaming them off a web server. (Alas, no flac suport.) It also handled mpeg4 video, although it felt like it was near its limits there. It has a Youtube app which works fine.
The not so good: it can stream mp3 radio stations, but doesn't pick out the "now playing" metadata. It also can get confused moving from Wifi to cell data and back, necessitating a reboot or at least a trip into "Airplane" mode and back out. The XMPP Jabber app "Loqui" is decent, but often ends up offline even though data service is available. Again, I think it's the transitions in data service, probably not the app's fault. The phone understands having your phone rotate and flipping the screen, but sometimes this just stops working, so you get to see everything in portrait. Even a reboot doesn't clear this reliably.
And probably the biggest red flag I want to wave: battery life. Is it because this phone is based on Intel CPUs? I don't know. But I'm down to my last 1/3 of battery life--best case--by bedtime. If you use the phone aggressively and don't charge during the day, I doubt you'll make it all the way through. For me, not such a big deal--I have multiple opportunities to plug it into a USB wall wart and give it a boost. You may want to go hunting for one of those super-battery packs which bulge out the back of your phone.
Overall opinion: Firefox OS is a tidy little product, leveraging many powerful technologies in the world of the web. When coupled with a moderately powerful device like the Revolution, it provides a clean and responsive experience. There's no bloatware on the phone, and all the apps I've tried were well-behaved WRT what they displayed and what permissions they requested. When I look at the various status tools on my wife's Android phone, it becomes clear that the phone is doing many, many things on behalf of other companies and their interests. The Firefox OS phone seems like a much more old-fashioned approach of having the device serve only its owner.
It's a keeper.