Call of Duty isn't bad, Activision is.
I didn't play any of the originals and also skipped MW, which seems to be the best according to everyone's opinion.
But I had an absolute blast with WaW, probably the 2nd best / most fun shooter I've ever played in multiplayer. Also, it was the last one in the series where map packs were free on the PC, so definitely worth the money that I paid for the game. However, the single player campaign was a bit of a disappointment (thanks Treyarch).
MW2 had a better single player campaign, it was far more polished. I didn't play much multiplayer though. Probably because I didn't want to pay for map packs, but also the whole setting was a lot less appealing than that in WaW. However, what made it worth the money was spec ops - small side missions. I sank many hours into those, finishing them on Veteran (mostwere designed for two players, so doing them alone is quite a challenge).
Black Ops was definitely the poorest out of the ones that I've played. The story was critically acclaimed, but it was really just fucking stupid. The difficulty curve was horribly unpolished. I found the zombie levels shit. Played quite a lot of multiplayer and it's quite rough around the edges.
If anyone wants to give the games a go, I'd probably recommend MW2 for the single player content. I'd recommend WaW for the multi player, but I'm not sure as to how many people still play it. Overall, I'd give the series a strong 7/10 (maybe 8/10 for MW2) and not more than that. Not amazing games, but still decent.
As for map packs and DLC... I'll probably buy MW3 and if I like the game in multiplayer, I might buy the map packs. MIGHT.