You say OS maps: does it use OS Grid Refs as well as "long/lat" ?
Yes - on main screen. I can also recommend an App called 'Grid Reference' which does just what it says - gives you very clear OS Grid Ref, no more no less. Have kept the app as well as what AlpineQuest as it does a useful job.
When you say the full version gives access to OS maps, I presume you mean it lets you use the app to view paid for memory maps? Otherwise 4 pounds to access to OS mapping for the whole country is insanely cheap.
The 'insanely cheap' bit is the right thing - I couldn't believe it, I gave it a punt at £4, spent £8 on Quo maps at same time for tiny area of the Peak District, then downloaded far more onto my phone which was more useful, faster, and much less money.
I've downloaded loads of maps since in both 1:25k and 1:50k.
I always take a backup battery unit for the phone (like a Duracell pack for £25) but could do with some weather protection for my phone - don't want to wreck new HTC One S...
I have noted that there are some more rugged Android spart phones coming out now as well - could be a fantastic combination.