Best game I've played in my whole life.
Mass Effect 2 is a perfect game in the genre. I didn't find any bugs or major flaws (the only thing you could say that is a flaw is the member's AI, but still, it seemed to be tweaked somehow since the game shipped and I didn't notice any dumb behaviour from my party members).
The story-telling process in ME2 will hook you up from the start! You start as sheperd in your old normandy ship from the original Mass Effect. Then something happens and you wake up in this facility that is under attack: "Wow, what is happening? I need to know!!!" Prepare yourself to devote the next days of your life to a single game. It is amazing how ME2 pulls you in to the world created by Bioware. I found myself completely dazzled by the NPC's, dialoque and plot.
Technical review:
Graphics - unreal engine at its finest. This means excellent graphics, no slowdowns or random freezes (I never experienced a slowdown during the whole game). The client launches in 10 seconds without breaking your pc (I found this to be a major improvement since I was playing Black Ops at the time and everytime I logged in to the game, my computer would slow the hell down to the point it was almost broken (and no, my computer doesnt suck :p). Anyways, the game runs smoothly and from the start everything is very enjoyable to do. Even just running around exploring some worlds is a neat thing to do, because the scenery is amazing and there isn't any technical issue to hold you back on your journey.
Gameplay - The battles were improved over the ones ME (which I didn't like at all) and are now very fun. It's easy to give orders to your companions and equally easy to do your own powers. Enemies are smart and will react to various situations. Also, depending on the enemy species, you can expect different behaviour in combat: e.g. while a vorcha tends to use cover and recovering health when they're hit (they're the only species that recovers health over time) a Krogan will attack head-on (because of the high hp and defense). You'll need to learn what to do and react when facing a species, it will hardly be the same tactic over and over again. This adds to the replay value and to the enjoyment of battles.
Dialogue - It is awesome. It isn't just a sily way to advance the plot or to complete a quest. You'll actually want to listen to everything a NPC has to say and you'll select every single option (of the dialogue). The voice acting is tremendous! Good production values. As a sidenote, I romanced Amanda because of her cute accent :P
Plot - The best I've seen in a videogame. Totally believable (if you consider the context of the game - galactic universe and such) and you can make a variety of choices to change how the story develops. That's a nice thing and makes you feel important to the universe. In my opinion, ME2's plot is the best to appear in a videogame, ever. It dethrones MGS4 (will I get a bit of hate because of this?). But yeah, it is THAT good!
RPG or Shooter?
This is a hard one. ME2 has elements of both genres. Sometimes it feels more like an RPG as far as the story and universe goes (you even have a codex that needs to be filled by means of exploration and dialogue); has a level up system (untill level 50) and power upgrades. On the other hand, the battles are on a over-the-shoulder perspective with big bad-ass weapons Gears of War style.
So, the way you look at the game is your own choice. As for me, since I'm a RPG fan, I look at ME and I see more of a rpg than a shooter, but that's my personal opinion :P
What you need to know is that it excells in both genres. If you're an avid rpg player, you'll love it; and if you're an action junkie, you'll love it as well.
To sum it all up, if you own a xbox or a good computer, then there is no excuse to not play this game. It's a must to ALL gamers out there. And as it is coming to the PS3 soon, all PS3 gamers should grab in the first day of launch!