It is worth it for certain games. I too have my PC as the main gaming system, but i have my xbox for the awesome games that are only console or only xbox exclusive, like forza 2, gh:wt (i know it is PC now, but still better on console imo) and when they come out, forza 3, FFXIII and WET (i played the demo today and it is SUCH cool game, it is like once upon a time in mexico, but in china town and with a chick).
I would say it isn't worth it tbh. If you don't have one already, an xbox would be better, the online isn't expensive really, i mean, £50 a year i think it is, that is less than £1 a week for online that is better than the playstations.
I am not an xbox fanboy, i love the PS3, have played it excessively at friends and relatives, but the xbox IS better imo. It has the same games as the PS3, but they are better online because XBL is a much better system than the playstation one (i forget the name now
) aswell as the exclusive games being better and the palystation exclusive games not being worth buying the console for, again, imo. The xbox controllers are better too. They take a little getting used to because they are bigger than your normal controller, but then they work perfectly and have amazing battery life, the playstation ones though don't last as long i find and then when you plug in your foot long lead, you have to sit with your eyes on the screen of your 40 inch TV, it isn't practical and you can't just whack another battery pack or another couple of AA batteries in, you have to let it charge