Not sure, I'm one of the few that has never played it either, however after all of the reviews i've read and friends i've talked to i will definitely be purchasing the series.
The second one is, honestly, weird. You are a big daddy, that dies after been shot a couple of times. We all wanted to be an hard to kill, man-tank; and we got a human with a drill. There are, anyways, good plus: the setting is wonderful, the story is good and it can sometimes get very scary (both games).
I've only played the first one (and looking at previous comments maybe I should be thankful) and I enjoyed it. There are a few issues with the controls if you're on PC but other than that the story was good to play through and there were some brilliantly executed scare scenes. I would note the graphics look a tad dated but they're still pleasant.
As I've said, haven't played the second one so I decided to ask a friend. He sad it was good enough but you shouldn't bother buying unless you plan to be a fan of the series.