I hadn't read any Steven King prior to the dark tower series, but I'm quite hooked now (currently on book 5)
I'd say that the first one is quite hard to get in to, I found that a lot of it didn't make that much sense, and felt a bit too "drawn out". It took me well over 6 months to read purely because I couldn't handle too much of it at a time (sounds like I'm downplaying it, but thats my style of reading).
Second book got absolutely AWESOME, some really cool stuff in there. The third was pretty good too, but the fourth....oh my god, that is THE best in the series so far. The fourth one also took me a while to read, I started off not liking it too much but it grew on me.
As you progress through the series you find out bits of Rolands past, and then suddenly it'll click and you'll think to yourself "Ah, now I know what they were talking about in book 1", I've had at least 3 of those in the past few reading sessions.
Definately my favourite book series so far, just jump in and go for it.
They are apparently making it into a TV series + films in 2013. I've also heard rumours that an extra book is being written that will fit in between (between 4-5 I believe).
Go, for there are other worlds than these...