The biggest disappointment is lack of applications. So you can't get notifications for Facebook messages, etc. But that's ok, given that the phone is in a very early stage and Facebook haven't built a proper app for it yet.
There are some bugs which might be slightly annoying at first, but it's ok once you get used to it. The most frustrating one is the text prediction when using the keyboard, because it doesn't learn automatically and I am messaging in 2 languages... But I guess that can be counted as an edge case. The navigation (GPS) works surprisingly well though! Overall, it's worth giving a try especially given it's price.
I am quite excited about it's future. The code is easy to read and should be relatively easy to add new features / modifications yourself. So even if it won't become popular in mainstream (and it's really hard to win user share in the current market), it can be an interesting platform for hacking.
I wrote a bit more about it at http://blog.gedrap.me/blog/2014/10/19/a-month-of-using-firef...