Well, in my experience, the 720 is an excellent phone. I have used it for over a year and found it served me well. My partner now uses two 720s and she swears by them. That said the 512megs of RAM is a bit of a bottleneck, but since you say you won't be pushing the phone, you may just see some "resuming" screens, but not much else. Battery life is excellent and it is a tough phone - I had dropped mine a number of times and there was no damage to report.
Now, as for W10, it is possible that the 720 would get updated. However, not knowing exactly how W10 for phones would work, it is difficult, I think, at this stage to say exactly how the 720 would behave. It is possible that some features of W10 may not work on the 720, but this is mere speculation.
Having said all of the above, why don't you look at the 730/735. They are newer phones and very good for the price that they are being sold for.
01-06-2015 09:05 AM