Now keep in mind, when it comes to the PS+ membership, you get a number of goodies for your money. Free games (retail and mini), beta testing for games, demos, and more. There's really no good reason not to get a Playstation Plus membership unless you just can't afford it. However if you've got the cash, the service pays itself off in spades.
Right now, Sony is running a promotion until March 4 that gets you an extra three months of PS+ membership free when you purchase a full year. This promotion is also available for the recent PS3 bundles that have the membership coming with them.
To give you an idea of what free games are available, recently PS+ members have gotten this list of games completely for free (for the life of their membership): "Retro City Rampage", "LittleBigPlanet 2", and "Jet Set Radio".
There couldn't be a better time to get into a Playstation Plus membership.