SW 2015 is now out & up to SP1.1 & working pretty well. There are a few niggles obviously being such a new version. As far as I can remember SW 2014 SP05 was fine.
With SW2013 SP3 (not 100% sure) and above , customer needs to be on subscription to apply service pack. So in your case you'll have to be on subscription to install new service packs. BTW I'm running SW2014 SP 5.0 and it works fine for me.