With Battlefield 2142 soon to arrive on store shelves it seems to have brought up the common debate of evolution vs. revolution in game development, and the ultimate question of whether a game is different enough from its predecessor to warrant a purchase. It's unclear what the answer is for Battlefield 2142 and so you can be sure you'll find gamers arguing for both sides on just about every BF2142 message board. I personally think that DiCE and EA know exactly what it is they are trying to achieve.
If you are familiar with the Total War series (Shogun: Total War, Medieval: Total War, Rome: Total War, and soon Medieval II) then you probably know that they are wildly successful and critically acclaimed by more than just strategy gamers. The Creative Assembly (the makers of the Total War games) attributes a lot of their success to their cycle of evolution, revolution. Shogun was a revolution as it was the first entry in the series and really a unique experience. Medieval was an evolution that improved that gameplay, but kept most of the core engine and mechanics the same. Rome was another revolution that completely redesigned the engine and the mechanics of how the overall game functioned.
This kind of cycle allows the developer to come up with innovative new ideas while at the same time refining previous innovations to be more functional, more balanced, and above all more fun. It is my opinion that this is what EA is trying to achieve with the BF series. BF1942 wasn't the first game of its type, but it took innovative ideas from a number of other games and managed to meld them together into something that was truly unique. BF: Vietnam evolved the gameplay somewhat (mostly with vehicles) but remained mostly the same. BF2 was a clear reinvention of the franchise, and BF2142 is the natural evolution of that game.
Does that mean BF2142 is worth your hard earned $49.99? Well that depends on your own feelings, most of all do you like BF2 (which I do) and do you like the new semi-futuristic setting (which I do)? Additional changes which may be important include: a more fully-featured squad mechanism, a new gameplay mode (Titan mode), and supposedly a much better frontend network (if you've played BF2 you know what a pain it can be to connect to a game sometimes). As for me, my inclination is to believe I will buy BF2142 and enjoy BF2142 (I love BF2 and I love mechs), but you can be sure I will be downloading the demo to be sure of myself.