Just played the first 3-4 chapters, so I guess about 1/2 way through, I was playing mostly sneaking like the first one which drags out the play time somewhat.
It's very much like the original in a "more of the same" kind of way, which is a great thing because the original was extremely well executed, everything awesome makes a return, the perk system, unlocks, secrets, and most importantly the styles of play, either sneak around silently doing take downs, lean out from cover in a CoD style, or run in dual weilding like an old-school maniac, all options are supported and each have their own unique unlock trees which have requirements that are tied into that style of play.
The only mechanic that doesn't make a comeback is the dual story route that really requires 2 play throughs to fully experience, the expansion is a bit too short for this to work, we also see several new mechanics introduced almost immediately which is nice, it has something unique to offer almost straight away.
If you want a more in depth review then I suggest you read my long post in the thread for the original game as I cover lots of point there, suffice to say it's a very good game and the expansion so far is just as good, the pacing is great, the introduction of new enemies, mechanics and weapons is set really well, the locations look really good and the game looks amazing while running at extremely smooth frame rates (specs in sig, running at 2560x1600)
If you liked the original just get this, I think it's worth the launch price assuming the length of the chapters and overall game stays consistent. This was somewhat of a sleeper hit for me last year as I arrived late and it turned out to be a really tight first person shooter that was thouroughly enjoyable, especially from an old school gamer viewpoint, being able to choose your play style is very cool. You'd think that deviating from a single play style into 3 would really hurt the game because it's easy to mess that up, but instead of being a jack of all trades it actually gets each play style right to a degree that provides a compelling experience, not unlike the balance of stealth and action in Metro.
Shame there's little said so far in this thread about the expansion because this definitely deserves more attention.