The second generation (2007-2009) was a huge improvement over the first generation. I paddled both generations in the SuperFun and much prefer the 2nd generation that I still paddle today.
My kids (12/14) both paddled 2nd generation 2Fun/Fun and ended up moving to the 3rd generation (2010-2014) 2Fun/Fun. The outfitting was improved and the knee area was brought in a little, not as wide as 2nd generation - and that made a huge difference them (tall/skinny).
I paddled a 4th generation (2015- ) in the pool the other night. It's similar in size to the SuperFun and I liked it. I need to spend more time in it before I can determine if it's worth the expense to upgrade. It is definitely more of a river runner than the 3rd generation, better or worse depending on your preference.
I don't think you can go wrong with the 2nd or 3rd generation. If want to do more playboating go for 3rd gen. If you want more river runner and less play then go for 2nd gen.
$350-$500 for used 2nd gen
$450-$600 for used 3rd gen
Some shops may still have new 3rd gen 4Funs in stock at deep discounts with the 4th gen having been released a couple months ago. If you can go new at a good price I'd do it, since it comes with the warranty. A local shop here as a 2012 3rd gen Fun for $765. Jackson changed from crosslink to linear plastic in 2012 if I remember right.