4E is good for you if,
o You want something new. After 8 years with 3.x, you're feel fatigued and are stretching out for new d20 games (Arcana Evolved, Iron Hereoes, True20, D20 Modern) or even beyond (Shadowrun, Warhammer, Midgard, World of Darkness)
o Any of the following things happened to you in 3E
- 15 minute adventure day
- Over-profileration of spellcasters
- Combats being over in less then 4 rounds due to instant death effects or massive damage
- By-The-Book DM getting overworked from creating all those levelled or HD advanced monsters, or self-made ones.
- Character Creation took to long for you
o If you like a system where all classes have their unique shtick, but are all equally useful in combat.
o You want easy tools to improvise monsters and stunts without breaking the game
o "Story-Breaking" powers like Fly, Scry, Teleport, Speak with Dead are limited in use and are put at a distinct tier, so you know when you can switch the style of your adventures to suite such powers
o You want magical items to have less importance, or make easy house-rules to remove them fully.
o You always wanted a "Combat Leader" type character without spells like the Warlord.
o You like complex combat systems. (That relies a lot on special maneuvers and powers for complexity)
o You like a new cosmology, and find the ideas of the Feywild, Shadowfell, Astral Sea and Elemental Chaos interesting.
o Less (no) mechanical focus on alignments
3E is good for you if,
o You think you're happy with the supplements you got, and you think you will still get a lot of use out of it.
o You're interested in adventure material from a proven source like Paizo.
o None of the typical problem issues mentioned to 3E every proved problematic for your game, since you avoided it, house-ruled it, it felt right to you, or just never came up.
o You want to focus your games on Gnomes, Druids, Barbarians, or Sorcerers. (This will later be reconciled in 4E, but for the moment, it stays a limitation)
o You like character creation micro-management like juggling skill points to flesh out aspects of your characters
o You have modified the game to suite your campaign, or built a campaign on the 3E rules, and don't want to make a new conversion or start a new campaign.
o You like complex combat systems (that relies a lot on spellcasting for complexity.)
o You prefer the Great Wheel or have already homebrewed your new cosmology.
If you can't decide, try both and eventually continue using both.
My group will probably play a few of the Paizo Adventure Pathes for a while, despite being pretty interested in 4E.