So I just came back from seeing this.
If you hated the first one, you definitely won't like it.
If you liked the first one, then you still might not like this one.
One of the problems is that they needed to focus on it either being a prequel, a sequel or an interquel. I mean it was fine to have a little overlap, but with a movie like this they really should've focused on one of the three.
The second problem is they didn't go over the top enough which was odd since it sort of set the stage for history to go flying out the window (which it did anyway) and they certainly had some really cartoonish things like Xerses' dad sitting on a throne of skulls (and a whole throne room adorned with them) but why hold back on having the monster factor like they had in the first one? Most of the battles took place on the sea so it really would've been the perfect time for the Persians to "unleash the Kracken" and have some sort of fishmen demons or something for the Greeks to fight. Instead everyone was just a boring human and they half assed everything.
The main hero was okay, but he wasn't anything special and that was sort of the problem. He didn't quite give off the larger than life hero aura that Leonidas did. Not to say he wasn't doing 6 impossible things before breakfast, but it was all just kind of "Okay, we see you're a badass, everyone is in this movie." (He does somehow ride a horse under water at one point though)
The female general had her moments and her armor for the last battle was suitably over the top with the whole dragon spine spikes in the back, and she was certainly blood axe crazy in battle, but I just got the impression they only had her in the movie at all to keep the homoerotic level at a mere 99% rather than 100% (The hate fuck scene between her and the hero).
Anyway, I didn't mind the movie, but don't go in thinking it is going to be better than the first, assuming you liked the first to begin with.