There's plenty of reviews/info about HEM1, so I'll just cover the new stuff
Vs Hero
This is pretty good. You can click on a villain while playing to see several key stats broken down into 'total' and 'vs hero', so you can see if someone plays back at you a lot (e.g. their 3bet% is 5, but vs you it's 8).
HUD/Popup Designer
A nice WYSIWYG editor makes customising the HUD and popups way easier.
Mini-charts
Option to replace the hudstats with graphs, seems a bit gimmicky to me.
Opponent Line-analysis
This is pretty sick. You can take an opponent and then do all sorts of filters on their lines. So, if you want to know what hands a particular reg takes a bet-check-bet line, this will tell you. You can go really deep on this in the specific Opponents tab, but rightclicking a player at the table brings up a scary amount of info. If villain cbets an acehigh flop, I can rightclick and bring up a piechart of what sort of hands he cbets on that sort of board.. does he check back TPWK? does he fire with JJ? Now I know.
iphone app
Seems gimmicky, maybe will improve with updates. Probably cool if you tweet HHs.
HM Sync
Let's you upload your database to HM servers. Really useful if you play on multiple computers.
Widgets, HMTV
Mostly RSS feeds of poker news, good if you're a NVG'er, but otherwise probably not that useful.
New stats
The amount of new stats is pretty unbelievable, though I wouldn't say that there's any real game-changers, but pretty much everything is covered (if you REALLY need to know how often UTG 4bets vs SB squeeze I think there's a stat for it
)
In summary, HEM2 is pretty awesome. There's some new stuff that's kinda pointless and superficial, but the line-analysis tools and the improved HUD & popup designer makes it worth upgrading for.