I've sat in 340 a number of times as well as other various 300 level sections.
The mezzanine level isn't nearly as bad as what Islanders fans make it out to be. Sure, climbing to your seats is annoying, who gets super excited for a game when they know they have upper level tickets sitting in their pocket?
340 overlooks the visiting team's attack zone so if you're a Rangers or Devils fan, you may enjoy these seats as opposed to sitting further down. It's easy to follow the puck which is what's most important. When these sections fill up, you'll find some rowdy Islanders fans but sadly, they don't show up too often to that extent.