So, is it worth it?
Very Yes.
Since its from the same developer (i think), is the gameplay similar?
I'm not sure, having never played Baldur's gate II. From playing Baldur's gate I, though, Fallout 2 was much more non-linear and open.
How does sneaking work (i aways play a rogue type character)?
Sneaking prevents enemies from seeing you. It's based on your sneak skill, light, facing (I believe), the armor you are wearing, and distance from enemies. There are perks that boost both your sneaking ability and your ability to do things (like run and do damage) while sneaking. Shooting guns (and running without the "silent running" perk) will de-sneak you, melee attacks that kill your target will not.
Is it possible to snipe things from afar ala jagged alliance?
Having never played jagged alliance, I'm not sure. I can say that long-range shots to the eyes with gauss rifles are beautiful things, especially with high luck and the sniper perk.
Is it nescessary to have an actual party, or i can choose to play with only my little vault dweller?
Nope, you can go through the entire thing without any party members. I think that it is possible to complete the game by killing every man, woman, and child that you meet. Obviously, some quests will be unavailable if you do that, though.
How long is the game, in hours, playing on normal difficulty and not wanting to get every single thing in the game (which i will, if i like it)?
A while. The main quest itself (if you know exactly what you are doing) can be completed in about an hour, but you won't do that. I'd say 15-20 hours at minimum, quite possibly much longer depending on which and how many of the major quest arcs you get sucked into.
Does my character's animation actually changes when he's sneaking (sort of a random question, but i hated how my character on baldurs gate just went semi transparent and stood there with the posture of a tango dancer while supposedly sneaking)?
If I remember correctly, you don't even go semi-transparent. Sprite space was at a premium in those days. The Fallout sprites tend to look a bit furtive anyway, though.
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