We are a nursing home with a newly-installed (by me) FreePBX system. We have 4 deskphones, 9 SIM-less Android smartphones and 15 iPads. We're currently engaged in implementing the iPads as comms devices, using CounterPath Bria. Right now the Androids are running CSIPSimple but I'm going to change them to Bria to minimise support/education - plus, Bria has a better integration with a PBX (e.g. it has a Transfer button, which is hidden in a sub-menu on CSIPSimple). Bria is a very clean piece of kit for very small money.
We also have two Unidata ICS1000G units - we use them for "parked" incoming calls for patients. They perform very well in that role - not super-robust and the menu system is a little obscure but, once configured, they do the job just fine.
We have a lot of trouble with the smartphones going "unavailable" as they move through the premises (for between a few seconds and a few minutes). I'm working on solving this - I believe it's because SIP is performed over UDP, the default setting on all softphone apps. If you're just using them within a single wifi environment then it makes sense to switch to TCP for the SIP - my tests so far are encouraging. There is also a fundamental difference in the internal wifi handling between the smartphones and the ICS1000G units - the smartphones will hold onto a poor signal long after a better one is available from a closer AP, whereas the ICS1000G will jump to a better signal much quicker. This makes a big difference if your users are constantly moving around, as ours are. Our Ubiquiti Unifi AP heads can apparently be fine-tuned to improve this irritating behaviour in the smartphones, but I have yet to experiment with that.
As to ease of use - well, a good SIP app on a smartphone looks and behaves pretty much like the built-in dialer on any smartphone, so it will feel familiar to most. Plus, of course, you can install a SIP app on your regular smartphone and it will auto-register to the PBX as soon as you come within wifi range - no need for any new phones at all, perhaps?