@Woffle99 - I was highly skeptical about the tactical battles... didn't like some of the ideas on paper, but they really are way better, really fun now... much more so than FE. There is actually some tactics to it now, rather than just run up and bash. You find yourself paying attention to maneuvering now as you try and make use of the special abilities. So big thumbs up on the tactical battles. This is probably the best thing about the upgrade actually.
It looks much nicer. That's not a huge deal to some, but it really does look significantly nicer than FE. (My only gripe is there is no groundcover but i'm hoping we might get an option to enable that in future).
The Fame mechanic for recruiting champions works pretty well, needs maybe a bit of fine tuning but it definitely works better than the random champions all over the place in FE. Having said I would mind seeing the odd champion out in the wilderness. Still, it's good.. better than in FE in in my opinion.
For me the jury is still out on the champion ability trees. I think they have potential but just need a bit of fine tuning.. I think it can work well once some tweaks are made.
There's quite a few little things that are just nice to have... movement paths displayed before you move, ability to upgrade the size of a unit, some nifty new spell effects like the wall of fire (line spells) that sort of thing.
Plus, the expansion will get all the love now as far as patches and updates go.
I know there is quite a few bugs being reported, but it's pretty solid in my experience... If your're a rabid TBS fan, particular fantasy based.. it's worth the upgrade. If you're not so much in that category, maybe wait and see if the upgrade price drops in future... or wait until they tidy up some of the bugs and tweak the champion trees and see what else gets added to it.