I've been a big fan of the series since the original Jagged Alliance, and JA2 was one of my favorite game experiences. I've played about 3 hours of JA:BIA so far so I'll give my reactions for those interested.
I found the demo way too advanced for my tastes so I didn't spend much time with it, hoping the game would ease me into the experience better which it does. So on to the game itself:
First I'll address the change to real-time plan and go system. While I've always been a big fan of turn based games, it's a gameplay style that just does not work very well anymore. The new system works well and keeps the game flowing at a faster speed. There still remains plenty of room for you to use strategy, and you can pause the game at any time and queue up actions. It has a bit of a learning curve and the mercs can sometimes seem a bit sluggish (the agility stat actually increases mercenary reactions) but the more I use the system the more I like it. While there will undoubtedly be many fans of the original who simply refuse to accept the real time system, turn-based systems are much more limiting and I think this new system is needed for this type of game to evolve and survive.
The fog of war removal seemed like a really bad idea originally but it hasn't bothered me as much as I thought it would. It's nice not having to explore every inch of the map to find the enemies. The biggest issue I have with it is that a lot of the tension of storming a building is gone. While you can't see inside a building unless a merc has a view of it, you'll always know how many mercs are in there and you don't have to see in every room to know the exact spot of the enemies, just one person at a window will give you a full view of all enemy positions. It's easy to overlook the no fog of war outside (maybe you have some sort of aerial reconnaissance) but the no fog of war in buildings is definitely a bit of a letdown.
The game starts out much easier then the demo, and has a tutorial which helps with the basics. Like the other games you can only afford a few of the cheaper mercs at the start so you'll start out pretty weak, but your opponents will be as well. Your mercs gain xp through almost all their actions (doing damage, killing, healing, repairing, disarming explosives, etc) but it seems to take a lot of xp to level up so it's hard to tell exactly how effective that system will be at upgrading your team. It took me 4 combat zones to get my guys up a level, which gives you 7 points to distribute among your stats and skills but with 10 stats/skills total you'll definitely be specializing your team rather then making jack of all trades.
The biggest problem I have with the game so far is that a lot of the personality seems to be lacking from the mercenaries. It might just be that the members of my squad don't have much opinion of each other, but so far there has been no interaction between my squad in the field. The only time I've seen an inter-merc connection was when I recruited my latest Merc and he mentioned that since he had heard good things about me from his friend (another member of my squad) that he would accept my recruitment request. I'm still very early in the game though so we'll see how things evolve. There is a morale meter, and each merc has traits that can change that so I might try to get some of my mercs morale low to see if the personality comes through more. Also the character portraits are pretty bland. They are 3d pictures of the characters faces and change based on equipment, but honestly I'd much prefer a good 2d drawing that showcases the mercs personality better.
Back to some general game observations:
There seem to be a decent weapon selection, but a lot seem to have similar or uneven stats (for example my shotguns and pistols don't have that much of a range difference from my rifles so far. Also I'm sure it must be a bug, but all my equipment pictures are really low resolution and blurry.
The camouflage system seems interesting. There are 4 types of camouflage (urban/jungle/etc) and different clothing and weapons will have upgrade your camouflage in different areas. There is also a visibility bar and noise bar right on your character portraits so you will always know how effective your stealth is. None of my characters are particularly strong at it yet though so I haven't really been able to explore the system fully yet, but it seems interesting.
I'm far from making a final judgement on the game, but it has some promise so far. It's not going to please everybody, and if you are a fantatic who doesn't want any change then don't even bother. If you want to see how this type of game might evolve then check it out. It has some interesting systems, but the merc personality plays a big part in why I like the series and I really need to see more of that before I can recommend the game to JA fans or not.
EDIT: A few things I forgot to mention in my first post.
As pointed out by Dredlockz there are numerous autopause options in the menu which makes the real-time elements much more controllable.
Also, inventory management is a pretty big pain. There really needs to be an easy way to manage inventory on the main map screen. I think you should be able to trade between mercs, arm militia, and sell items from the world map but so far I have not seen a way to do any of that.