For SFNE specifically, just get there early and go straight back to Bizarro. Ride in the front a couple times while the lines are short then switch to the back row for the remainder of your rides. After Bizarro, head to the Batman coaster (which different from other Batman coasters at Six Flags). Everything else in the park is mediocre at best, so ride attractions with waits of 20 minutes or less, skip everything else.
Food is subpar and expensive.... so plan on bringing snacks or leaving the park for lunch.
SFNE is not a bad park overall, in fact I had a pretty good time when I visited in 2011. But it is Six Flags, so don't set your expectations too high.
TPI's page on SFNE is mostly up to date... check it out if you want some additional info on the attractions at the park.
As far as coasters go, Bizarro and Batman are the top two. You'll want to do both of these within the first hour the park is open, especially if you aren't planning on getting a Flash Pass. Definitely try to get to the park 20-30 minutes before opening so you can be one of the first to Bizarro, as it may only be running one train in the morning (on my visit, the second train wasn't added until the afternoon despite a 90 minute wait). After Bizarro, ride Gotham City Gauntlet Escape from Arkham Asylum if you wish to ride it, then go do Batman. I'd recommend skipping Catwoman's Whip and Mind Eraser unless you're trying to ride all the coasters.
Once you're done with the above, head to the north end. The best coaster up here is Pandemonium, but it has low capacity. Cyclone and Thunderbolt are decent wooden coasters, but both often run only one train resulting in slow moving lines (on my visit, Thunderbolt had one all day with a 60 minute wait, but Cyclone was using both). Flashback is a very rough boomerang...skip it. Goliath will likely be closed, but if it isn't and the line isn't too bad it might be worth a ride (it ran pretty well at SFMM, but I've heard it's been running worse since the relocation).
As far as non-coaster rides go, I'd definitely recommend trying Houdini and Scream. If the weather is decent, Blizzard River is also an excellent water ride. The park has a number of other flat rides, but I was too busy riding coasters to bother with most of them. Most of them didn't seem to get too long of a wait, so you could probably ride your favorites whenever. Unfortunately, my favorite (Catapult) is no longer at the park.
As for Flash Pass, I don't know how crowded it will be in May versus July, but the park is small enough that unless it is busy you'll be okay without one if you visit for a full day. In July it was necessary (all major attractions had 60+ minute waits), but if everything was below a 45 minute wait I wouldn't get one. If for whatever reason you are visiting for only 2/3 of a day or less, you will need one to get everything done.
Overall, Six Flags New England is a nice park. It isn't a great park by any means, but it is one of the better and more balanced Six Flags parks. As long as you go with that expectation in mind and aren't disappointed by only one headliner coaster, you'll have a great time.
