I loved L4D I was addicted to it. All the characters were likable, the special infected are great and the level design along with the music was awesome.
I hated L4D2. The characters were boring as were the levels and the new special infected, and the music just sounded awful, the only thing I liked about it was the improved graphics (mainly the way the zombies fell apart) and some of the melee weapons, I also disliked how impossible it was to win on online versus, me and my team would always win on L4D, we were awesome, we only ever lost to teams who like us had been playing together for so long that they could play without mics, but on L4D2 we got beat all the time by randoms, I hear it was designed so that the humans barely make it, but that just took the fun out for me.
I would buy L4D3 if it was really cheap and I had heard good reviews along with positive word of mouth from other gamers.