So, I used to play Warcraft III. I never played the first starcraft. I am thinking about buying Starcraft II. How does the playstyles differ.. will I be able to easily adjust to the Starcraft playstyle, or will I be disappointed because it is unfimilar and not what I will be wanting? I'm wanting to play primarily custom games - do you play the same sort of thing, hero wars, tower defense, etc etc?
as far as custom games are concerned- you will see similar things, but the scene is in its infancy at this point so there are some cool games out there but many many more are still in the works as far as general gameplay is concerned- I played both and warcraft 3 is more micro intensive and smaller scale while starcraft 2 has a balance of micro and macro and is larger in scale. But both have that blizzard RTS feel to them.
One very important aspect which you'll be missing in SC2, are of course the hero units. There are obviously some very powerful units in SC2 which will be able to turn the balance of a battle in your favor, but it won't ever be as powerful and important as the WC3 heroes. (for a single unit) The biggest difference between the two will have to be the duration of battles. In SC2 and it's predecessor, the units die fast when countered and approached in the right way. And with fast I mean within seconds. WC3 units have a considerably longer lifespan then SC units, mainly because SC doesn't really have that many self healing and snares/buffs from units amongst each other. The game still offers a ton of opportunities to micro your armies, though. There are plenty of abilities for each units, both researchable as well as standard, so you won't be short on that/ A simple attack + move won't get you the win in most battles. ^^ Other then that, the upgrade and supply systems are very similar, while all SC2 races also have very different playstyles. Even more so then the races from SC:BW and WC3, actually. And as Snowden already mentioned, the mod/map community is already working in overdrive to create the best new custom content out there. (can imagine a lot of WC3 talent now starting to work with SC2) If you're into the style of gameplay which WC3 offers and you don't mind switching to a more speedy sci fi style, then your money will be well spent on SC2.
If you are mostly interested in custom games then get it. First, you will see a lot of the same game styles with new units (more sci-fi theme obviously). Second, the map editor is supposedly really powerful, and from what I understand "hero" units (i.e. the 'leveling' mechanic) are possible and will likely be seen in SC2 custom games. As for the 'mainstream' pvp game: The pace is far faster than WCIII and there is a bit more emphasis on macro imo. I've played all the Blizzard RTS games and after a multi-year break I tried SC2 beta, then when beta went 'dormant' for a shortwhile I dusted off and re-installed WCIII to get my 'fix'. IT - FELT - SOOOOOOO SLOW. Both in terms of base construction and in battles (i.e. units take a lot more time to kill). Still, I think as long as your main appeal for one game or the other isn't the background setting (Sci-fi vs fantasy) you will like either game. That goes even more so if you mostly enjoy custom games.
Heroes are REALLY powerful in WC3 if played correctly. Once you get them to lvl 6 and up (which is easy to do in a standard game) they are so strong that battles become "kill the hero" with the actual armies becoming more like sidekicks. Still, it's a funny game to play with your buddies, especially the custom maps. I'd get both.