I actually beat the first and second for the very first time within the last month or so. Excellent games. With that said, everything you've ever heard about BioShock 1 is absolutely true, both good and bad.
The game thrives on the atmosphere of Rapture, the plasmids and gene tonics add a nice twist to the shooter mechanics, and the game is full of jaw dropping twists. Unfortunately the negative criticisms were true as well, the second half of the game drags a bit, it isn't quite as fun as the first half and the ending is very lackluster. Regardless of which ending you get, it is too fast and too vague.
Onto BioShock 2. Without spoiling anything, I adore this game. Perhaps more than the original. There are two important things to note: 1) The atmosphere isn't as wondrous the second time around. You can't lose your Rapture virginity twice, unfortunately. And 2) while the second installment doesn't contain as many shocks... the plotting, pacing, end game, and endings are all superior to the original.
What you do get beyond that, however, is a vastly improved and tweaked experience. Every single element of the game has been refined. Ever find a room too dark to see in the original? In BioShock 2 you have helmet lights that will turn on or off depending on your lighting needs. Tire of switching between plasmids and weapons? You can now dual-wield. Also tired of switching between guns for ranged and the wrench for melee? All guns now also work as melee weapons.
The list goes on and on, there are hundreds of tweaks that may seem inconsequential, but they all add up to a more enjoyable experience from a gameplay perspective.
