I can't see the need to be a PLUS member right away - almost nothing is avalible to download on the PSS Plus section. But if you want Little Big Planet then u should maybe go for it? (it's a free download for new Plus members).
Im gonna wait until it gets more meat on it's bones - so to say,
at the moment I can't see any other reason then to get Plus just to show Sony your support - atm that is! Im sure it will grow in the next couple of months. But have in mind that in the next couple of months you can pick up some great titles that are being released.
inFAMOUS 2, Killzone 3 (with move support), Dead Rising 2, Co

Black Ops, Driver, GT5, etc etc it's a long list and many games are coming out very soon. And to that you can also add the release of the MOVE controller and all it's games and accessories - if you're into that aswell.
So my conclusion is: Wait some more and save your money.
Worst case scenario is that Plus is getting super awesome at the same time of all these releases - but if you have your $'s then get it all at once. Or yea of course - get it now if you want to. But I would rather take a great title to play many hours on - then to get Plus at the moment. Wait until it's really worth it.