I wouldn't bother if you already have W7HP.
W8 pros:
1) The ability to combine drives into one big drive pool.
2) Far better licencing terms.
3) Decent UI for touchscreen.
4) Latest and greatest everything.
W8 cons:
1) Edge Swipe is terrible for touch pad users (can be disabled).
2) Plain 2d appearance.
3) If Metro was well designed you could just use it, instead one must learn how to use Metro.
4) No Games unless you sign up online.
5) M$ is forcing you into their "app space". Gadgets are gone because of "security".
Metro you CAN avoid for the most part especially if you customize your PC/Laptop desktop with shortcuts etc.. you don't need a third party Start Mod like some users have ie Classic Shell but I can understand some users are more comfortable with one,as to no games well there are plenty of free ones you can download the normal way like you do with Win7/Vista with browser etc.....so don't even need to go to Store,I'm a gamer and get my games the normal way ie via Steam,Origin, game websites plus free downloads from sites etc...
It boots faster and IS more snappier then Win7,better security,better battery life for portable gear ie Laptops/Notebooks/Tablets etc, improved Task Manager,Refresh and Reset options ,I could name more but you can see the rest here
http://www.pcpro.co.uk/features/374587/30-best-features-of-windows-8 .
Bottomline try it for yourself(ie free Preview version) is my best advice and use it for a few weeks to really get use to the changes.
yehness I will say about your XP statement nothing wrong with Vista, Win7 or Win8 with regards to stability so XP was not the last stable OS.
As to good/improvements well it depends on what you are really looking for (we all like different things) ...I've been happy with most of the Windows Operating Systems ie XP,Vista,Win7,Win8.
Plenty of threads on Win8 from both sides,end of the day ONLY YOU can really say if its worth it.