Sadly, it's rather inconsistent in tone and quality, and it runs the gamut from being (or attempting to be) dramatic, humorous, suspenseful, creepy, action-packed, and fun. Sometimes it's effective and engaging. Sometimes it's painfully cheesy. And sometimes it's strangely "why the hell am I even watching this?" boring.
The first season was easily the best, because they managed to balance all the aforementioned elements of drama, humor, suspense, atmosphere, action and fun. The second season...I don't know what the hell happened - as near as I can tell, it was still mostly written/directed by the same core people - but it really slumped in quality. Hard. The majority of the episodes were just...dull. Several episodes had intriguing ideas, and should have been great, or at least interesting, but they seemed to lack the same vitality as the first season. Truth be told, I had to push myself to keep watching, because there were very few episodes that made me excited to see what came next. (Plus, and maybe it's just me, but the whole 24-style multi-window/comic book panel thing got seriously annoying. Like, they only just got this new technology and they had to use it multiple times for every episode, because someone thought it'd create tension and drama when, in fact, it had the exact opposite effect.) The third season got better, particularly toward the end where most of the episodes were following the same story thread, which was compelling enough to make me eager for the 4th season. But I still don't think the show is living up to its potential. Which is disappointing and more than a little frustrating.
As for what I'd compare it to in terms of quality... Same ballpark as Stargate or Torchwood, maybe? In other words, you never know what you'll get from episode to episode. When it's good, it's really good; when it's bad, it's truly just awful.