Yes, it's worth buying, it's an excellent game, relatively comparable in game-play mechanics to ME2, it has its own unique ones. It's about the same length (dependent more on the difficulty setting than ME2, in my opinion). The dialog quality (and amount of it) is also comparable to that of ME2's (and ME1's), sometimes better, sometimes a bit "tasteless" (although rarely so). There's no useless scanning for resources (artificially inflating the game's content or length, for example), there IS scanning though, but it's not "useless" as much nor even comparable to what it was in ME1 or ME2.
There's more "main quests" than side quests in this one, but there's no "landing on 'x' planet for N7 Assignments" types of missions that isn't going to benefit the "longer term" main quest. For example, side quests directly help the main quest in some ways, contrarily to just be side quests that don't do much or if anything like in ME1 or ME2... for example landing on a planet just to see a Mech firing at some rocks while powering it up with batteries along the way only to gain some resources... there's no stupidity like that this time around.
There's a few issues here and there, some bugs, some camera-related problems (dodging, getting blocked), but they aren't frequent and they happened in ME2 as well (floating, getting blocked, etc). I personally don't like the quest-tracking system much but it's a question of adaptation more than anything else. The actually quality of game-play is great, it surpasses most of (not all of) ME2. Game-play-wise specifically, yes, an absolute blast, not a perfect game (none are), but much better than the "usual" generic crap we have to deal with, still by now considered to be one of (if not "the") best BioWare games (personally I think it's their second best, after KOTOR, don't get me wrong, I really like ME3, it's epic, generally speaking, but KOTOR had a much profound impact on me than ME3 did even after the first-playthrough, but that's subjective).
Story-wise (NO SPOILERS)... well, let's just remind people (generally) that the ME trilogy has been written by a total of three different writers, one of which did most of the work since the release of ME2, namely Mac Walters (Drew was the lead writer for ME1, then co-wrote a bit of ME2, and supposedly had "some influence" for ME3's story as well, but hasn't been proven). It is clear to me that despite obvious attempts at "connecting all the dots" properly ME's lore stability and cohesion isn't perfect. It's a very complex sci-fi epic, and like others in its category it suffers from unexplained/incomprehensible/lost information (or plot holes if you want). Most of ME3's Codex information and in-game events, along with characters behaviors, dialog, past history revelations, etc, does fit "well enough" (sometimes very well) with regards to what "we know of" from ME1 and ME2.
Most of it all does fit somewhere in the big picture, but there's still many unanswered questions, some of them very important (which is creating a lot of discontent amongst the community, with reasons). Regardless of the story/lore-related issues, the actual GAME, the game-PLAY, the entertainment of ME3 is irrevocably grand.