I've never played Borderlands, and right now it's $10 on steam. 
The Giant Bomb guys always seem to insist that 360 is the best way to experience it.
So - should I make use of the offer or wait till I find it for cheap for the xbox?

Yes. Yes you should get it.

It's definitely worth buying at that price, but personally, I would wait until they make the GOTY cheaper. If not and you want to play it now, my suggestion is get the normal version and pick up Knoxx with it (it's an amazing piece of DLC, which almost feels like a mini-sequel).
 
Personally, I think playing this with a controller is silly. You'll really want the accuracy of PC controls, especially for getting critical hits, which can be almost essential for hard enemies (enemies can sometimes take many critical hits before they actually go down). Also, you can disable all the unskippable cutscenes/intro screens in the PC version, which became quite annoying.
 
However, keep in mind that if you're playing co-op, the PC version requires that you use Gamespy. Other than that, I didn't really find any issues with the PC version (well, maybe there could have been a few more keyboard shortcuts...but that's really it...)

hey ppl. i've been also thinking of buying this off of steam, but I want the game of the year addition since it comes with all DLC packs. A question though: how is the co-op on pc? i imagine it's way easier on consoles since they have a certain social focus, but is it easy to find some awesome dudes to play with (especially a year after the game was released) or do I have to hope friends, who have awesome pc-s (none of them do) and who bought borderlands (none of them have) are also playing this?

Get it! Bought it on a whim during the Christmas sale along with all the DLC. One of the best decisions I've ever made.

get it on 360 or ps3 preowned :) 
might cost 5 to 10 more but you will have a better time with it 

Definitely.

Great game, definitely should buy it on some format or other. I can confirm it is better on Xbox (at least the controls are optimised for gamepad and XBL is better than Gamespy), but not by much. Play with an Xbox pad if you have one. Also, whichever system has friends playing if applicable.

I have it on PC. I dont know anyone else that does so I never got far in it. The reason the Giant Bomb staff say its better on consoles is the UI (menu systems and such) are not ported well for PC. They were made for console and on PC feels really clunky. Other than that the game is fine. Totally worth getting.
 
Also you can get the complete pack which has all the GOTY stuff (I believe anyway) for 10$ less than the GOTY edition. Just sayin. Well that one ends in a few hours but, ya.

I can trade you a copy for something,i bought the 6-pack and still have 2 copies left over.
 
Edit:Down to 1 now.

I have it on PC, and not having played it on console I have no direct comparison, but it's fantastic. I'm playing it with a gamepad. 

Play with a gamepad, it's fine.