Logitech MX518 or Optical Mouse?

My current Logitech (from 1997) is beginning to fail me and this is
getting my parties wiped perhaps (couldn't drink a healing potion in a
dungeon last week because my left mouse button failed).

What I am thinking is that I'll just get a new logitech mouse (and
perhaps keyboard), but I want to know if anyone here has any experience
with the new game mouses from Logitech?

I am also not sure if I like the fact that it has more than three
buttons. Is there a use for this with WoW? Like maybe jump, escape, 'F1'
or 'F'?

And how about running it on Windows 98SE? (which I still use atm) G5
does support it, but the weights seem like a useless gadget to me (like
I will ever notice the difference and get it right)

I fear the gamer mouse is really meant for FPS gamers and not for 3D
MMORPG games like us. Still, I would value your opinion on this.
Especially from WoW players that use such a mouse on 2000/XP or 98SE/Me.

Life is like a videogame with no chance to win - ATR