I'll have to agree with most here that it is to soon to judge. From what I've read in the early 80's the only 757 orders were from BA and Eastern...now the 757 is the only mainline type that AA, UA, DL, NW, CO, TW, US and HP ALL operate (thats 8 of the top 10 US airlines). That is pretty impressive. So who knows what will happen, any of these types could catch on.
About the 717: I think its biggest flaw is commonality. Maybe if Boeing invested enough to make a shoter 717-100 and even a longer 717-300 (a new MD-80? please, please) sales would rise. It seems like all the other 100-110 seaters out there (A319, B735/736 plus A318) can operate much longer routes. The 717 can't, but I'm guessing it is lighter, cheaper and more efficient.
From what I know TW has 50 717s on order. Air Tran has 50, Hawaiian has 13 plus options and I'm not sure how many AB (Spainair connection) has on order. Olypic Aviation has 2 but I heard they were ordered for political reasons and they are kind of odd balls in the fleet.