I was fortunate enough to get a closed beta code, and I've played the beta for a few hours. I'll try to keep this relatively short. First of all, the beta features a single map, "High Tension", which is a map that supposedly takes place in downtown Los Angeles. There are also two modes in the beta: Heist and Blood Money. Heist is like a twist on Capture the Flag where the Criminals must bring loot to the escape point, while the Police need to prevent it. In Blood Money, two teams of criminals work against each other to see which can get the most cash. In both matches, the games seem to go by fairly quickly, meaning you can play more games in Hardline than you likely could in Battlefield 4 in the same amount of time. For me, that's a positive.
My first impression of Hardline was something along the lines of, "Hm, this is pretty decent." Hardline's color palette is surprisingly... bright? I guess? I've seen some people say the game looks cartoonish. Personally, I like this look, as it suits the more-arcadey-than-Battlefield 4 feel of Hardline. Honestly, I feel like it has a Call of Duty-esque thing going on, but at the same time, it still feels distinctively Battlefield... just a bit more fast paced. Confusing? Not sure how else to word it for you. I really appreciate the level of recoil on guns in Hardline -- that's what really makes it feel more like Battlefield rather than Call of Duty.
The map that's in the beta is alright, even if it's a bit uninteresting. It's a city with a few buildings you can go in, but it seems small. Since there are only 16v16 teams in the beta though (will it be like that in the full game?), the map feels barren in spots. If Hardline does support 64 players, it's going to be hectic to say the very least, as High Tension doesn't feel like it'd play well with that many.
While Hardline is like Battlefield in many ways (imagine that!), there are a few key differences. First of all, there aren't really any assault vehicles. Sure, there are a few turrets on vehicles that you can use, but there aren't any heavily armored assault helicopters or tanks rolling around. Second, you pay for weapons and gadgets more than you actually unlock them. I don't mean that you do this in a Counterstrike sort of way, because whatever you buy is permanent, not just for a single match.
I've enjoyed Hardline the bit that I have played it. It's obviously hard to form much of an opinion based on a single map beta, but it's a game I'd potentially purchase. Many are crying that the game should be Battlefield 4 DLC because it feels like a mod, but that makes no sense. Does Battlefield 4 not feel like Battlefield 3? Comments like that make no sense to me.
If you're looking for something a little different, Hardline could be a great option when it releases. It's Battlefield with a nice little spin on it. If the game runs as smooth at launch as it has on the beta, it may be worth being a day one purchase.