I've found that people that think IFE is a big perk don't like the MD series (for obvious reasons). Everyone who doesn't care about that seems to love the MDs (88/90/717).
And then there are some people that don't like them because they are "old" (not that this really means anything in my opinion).
The 717 and MD-90 are my favorite planes to fly in the DL fleet (technically the E175 is my favorite, but I've been on exactly one of those on a 30 minute hop from PDX->SEA so that doesn't count
). The MD88 seems to have worse seats so I'd put that in a second tier behind the MD90 and 717. The only downside is that they still don't have the faster internet which is an issue (although with lower pax volume driving congestion this is manageable most of the time).