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BUT, there will be a steady flow of quality games coming out over the next few months. I'd suggest investigating a PS2 game site, seeing what's available, and what's up and coming. If there's stuff there that you like the look of, which won't be available on one of your existing platforms, go for it.
I'm not being anti PS2 here. If you asked the same question, but without having all the platforms, I'd say definitely get one now.
If you are not a fan of any of the particular genres, then no, it's probably not.
No launch titles were exceptionally outstanding, but there are some very good, very solid games available.
Only you can really decide. Find a store with a spare/display/demo model (there must be some by now), tell them you're uncertain, and ask if you can try some of the games in the shop. (Obviously a dedicated games store would be best for this!)
That should help you decide if you really want to splash out on one. Personally, I don't believe that the PS2 is over-priced, but that doesn't mean that £300 (more with game and memory card) isn't a lot of money.