I've used it on my Nexus 4 and Nexus 7, and it's generally been a positive experience. I wish that I could disable third-party cookies, however.
Firefox is my main browser on desktop, and I go through times where I use it as my primary on my phone and tablet. It has some key advantages, and some weaknesses that make me come crawling back.
Firefox lets me disable third-party cookies, it has a great reader mode that allows you to read a less cluttered version of a web page, it supports extensions (though I haven't seen any for mobile that are useful, aside from AdBlock, which doesn't actually function), and I can change numerous
hidden settings through about:config.
However, it requires that I change a hidden setting (browser.formfill.enable) to disable form autofill and search history (Chrome has this in the settings manager), it doesn't have a tab strip on tablets like Chrome has, it's tab sync is far less reliable and usable, and it doesn't save my tabs if I kill it or turn off my device (probably the single most important feature of Chrome on Android for me).
The last one is planned for a future version, but all together, these pros and cons keep me oscillating between the two browsers on Android.