I recently (few days ago) got the base model 550 for my mom and updated to latest xxx.218 OS and xxx.41011 firmware released this month, there were some UI/functionality glitches and occasional app crashes but seems to be pretty stable after hard reset.
Of course this has only few apps being used like email, facebook, skype, whatsapp, Edge - no cortana being used here nor advanced stuff like Hello or Continuum available of course
🙂
Basic phone calling and SMS/email at least is stable - which is mostly what we need - we don't do fancy stuff on mobile phones.
Battery life with moderate usage is good (lasts around 2-3 days) but of course I've tweaked whatever settings I could to minimise background activity and data transfers.
I find W10M pretty smooth at the moment but not as buttery super smooth fluid UI animations like there was in WP 7.x and 8.x on much lower end devices like HTC HD7 (mine - single 1 ghz CPU core!!! - 6 year old phone!) and
Nokia Lumia 635 (Aunt's). People say it's smoother on the 650 model so will wait to see how that turns out if/when I get that phone for myself maybe in 1-2 months timeframe.
Overall I would still think the current released "stable" versions of W10M should also be considered in "Beta" state (performance and stability wise) but looking forward to the upcoming "Anniversary" or "Redstone" releases in few months time - then it should be much better.