I'm around Gambit Infamy Rank 10/11 so I've played quite a bit now.
I absolutely LOVE this game mode. I think the Primeval Slayer buff works perfectly. It may be harder if you're not in a team because a less co-ordinated strategy means you will struggle to keep off invaders and kill incoming blockers if you have spawned the Primeval first.
Sleeper remains the go-to gun for invading. You definitely can use and win with other guns but because it's super easy to use, once you have the option it's kinda just too easy to not use anything else. Bounties to use Rocket Launchers, etc do help to combat it a little. Let's see if the hit box reduction it gets will impact it when they eventually release it.
Definitely try to group up as much as you can - the only part that @Stretch616 said last night was about Invader energy against 4. 6 may be more beneficial. Hell, I would love to see more 6v6 activities but you I'm really excited for Gambit private matches which they've confirmed is coming soon.
Maps wise I love most of them and how the variants work. I don't like the Prison of Elders one, more because there's so many holes you can fall off it's really difficult to navigate. But as the enemies change it gives more variation to maps which you can take for granted e.g. when Tracer shanks spawn with Fallen, or Turrets with Cabal, having a player on Dreaming City map stand back high taken stuff out - @crispymorgan did this well with his trusty bow a few nights ago.
I'd go as far as saying it's one of the most exciting and refreshing game modes I've ever seen introduced into a video game.
Let's hope there is some contenders for Sleeper in the not too distant future.