If you play online or multiplayer games regularly than it is worth it no matter what your speeds are. I live in an isolated rural area and i am only able to get basic DSL they claim 1.5 mb down and 800k up but half of that is about average so 750k down and 400k up and the only time i run into lag issues is when i host a game. But that problem is easily solved by simply joining games rather than hosting them.
Lag however is always going to be an issue no matter what speeds you have, and don't let anyone say that this speed(cable) or this service (such as PS3) will do away with it. It is a part of online gaming.
Its kind of based on opinion. Like you will have all these fan boys saying yes. But i have a ps3 and it works awsome and my internet is really bad. My friend has a 360 and he has to have really good internet or he lags. So i would say no its not worth it. Ps3 is free for online so mabye try getting a ps3 idk. : 0