Pros:
- Great new campaigns that are much less linear than L4D1
- Awesome new weapons (AK-47, SCAR, etc) and equipment (laser sights, explosive ammo, etc)
- Great multiplayer modes and weekly mutations (custom game modes by Valve)
- Improved graphics and effects over L4D1
- Feels more polished than L4D1
- Excellent with friends
Cons:
- Overall game formula is identical to first game
- New Survivors are not as memorable as the original four in L4D1
So, is it worth $20? I would say yes, but I bought it on-sale for $10, and I would recommend waiting for it to go on sale if you can hold off on it.
But if you absolutely can't wait, buy it. It is worth it.