I loved Bioshock, but personally with Bioshock 2 I agreed with most commercial reviewing sights/magazines. The second time around is still good, but nothing is ever like the first time experience. When you crash land in the pacific ocean and enter the biosphere in that lighthouse....it's breathtaking what lies beneath.
Personally if you do buy Bioshock Infinite to get the first one for free I would definitely get the second one and play those two first. Although Infinite technically isn't related the first 2 are a must have experience.
But you'll do a lot of waiting, as Infinite got pushed back to February of next year. You can probably get 1 used for real cheap, probably $5-$10.