How is this game controlled primarily?
Old-skool flight stick like in microsoft flight simulator games, or mouse and keyboard control akin to descent series, or even freelancer?
Does a gamepad work with this game?

Yeah, required indeed. My HOTAS setup is currently packed away in a box on the other side of the world, so after downloading last night I (not very optimistically) tried to fly using only the keyboard.
One word: Impossible.
I'd have to either ship my controller setup here, or buy myself at least a low-end stick locally.

One of the ways you can still enjoy the sim until your joystick arrives is to use autopilot. This is the way I learned to fly in IL-2. I'd use autopilot to take off, noting what the AI was doing as far as flaps and speed. Once the plane was safely in the air I'd take over until I got myself into trouble then quickly turn over the controls to my "instructor".

I'm just wondering, can yuo use a mouse? I hav a joy stick, but I really like the mouse.

I just tried to set up the HOTAS controls to my mouse and it was a no go. As is, for me, I would find it impossible to fly with any control using a mouse

No. The mouse is used to pan the view and scroll the map.

Playing IL-2 without a good, precise flight stick is like playing an FPS without a precise gaming mouse. It just doesn't work, especially since IL-2 aircraft tend to be very twitchy and react strongly to small adjustments in stick movement.
Since IL-2's avionics are rather simple, you can probably get by with a relatively affordable stick-though, if you can afford it and don't mind the slight ergonomic quirks, I'd spring for the CH Products Fighterstick and Pro Pedals. Precise, reliable, and well-suited to this sim, though the Fighterstick digs a bit into my palm and the Pro Pedals are rather narrow.
TrackIR is also key, though. If you know where to look (such as Buy/Sell/Trade subforums on flight sim forums like SimHQ), you can get a 3rd-or-4th generation model for less than US$100. It will do wonders for your situational awareness and you'll soon wonder how you ever flew without it, not to mention that it frees up a precious hat switch on your stick (often the only one) for better things (say, guiding X-4 missiles).

Stupid question, but do you think I could play this game decently with my new Attack 3 stick? There's some sort of twist back of it and 4 buttons at hat.. Or does this need some more "pro" class stick?
I ask because I haven't played flying sims before (just some helicopters in OFP) and I don't know what they really require at minimum. I'm not thinking about any pedals or TrackIR (at least not yet, heheh).
I would like to test this one, been thinking about it a while now and it looks interesting, yet challenging.

When I got started in flight sims I started with a cheap stick that had un-programmable buttons and no rudder twist.. Flew like that for three or so months and had fun. Was having fun and decded a more expensive JS was worth it.
So you can look at like this:
Buy IL 2 (10 USD) and buy a "better" JS (45+ USD) and discover you don't like it, your out 55+ USD.
or buy IL2 (10 USD) and use your stick (free). If you don't like IL2, you are out 10 USD. If you do like, can decide how much the added value of the stick is

Thanks for your post, and sorry for my late response.
I had a chance to try my own stick with the original IL2 which I borrowed from my friend. I found this game quite difficult, but I think my stick will do it - for now, anyway. There's many stick options I don't even use, just learning basics.
You're right, maybe I should consider that more expensive stick afterwards if I get 'into the game'.
And because I missed that great holiday-sale because I wasn't sure enought for this title, I think I'll wait for next sale and get this gog's better IL-2 box. For now I think that old, unpatched, unmodded vanilla will do it. At least when I learn to get into air with every tryin'... :)

It's not impossible to fly by using keyboard, you just have to get used to it. I'm playing IL2 using keyboard for some time, and I started playing online some time ago, using HyperLobby and I can manage to shoot down guys who play very good, and they're using joysticks.