I finally finished the 6-man version of Forsaken Isle and can now confirm that:
If you like the beginning of the campaign you'll almost certainly like the rest. It's pretty consistent I think.
Truly a great experience, much better than the main campaign in many regards. I can count on the fingers of one hand the encounters that didn't pose an actual challenge, and they all take place when you're already at level 12 (the chunk of fights that goes from the witch hut to the final boss is a bit underwhelming, considering what you had to face just a little earlier). I limited myself to a long rest after every 3-4 encounters (except for when the situation explicitly expects you to immediately rest after a fight) and, even if it's definitely possible to rest even less, I feel like that's the sweet spot for me.
I played with a balanced party (judgment paladin, commander fighter, survival monk, battle cleric, balance druid, court wizard), without using content from the UB mod, and one of the greatest pleasure I had was to actually feel the impact of my character's features and abilities, something that's sorely missing in the official campaign, where you can clear almost every encounter by just auto-attacking and spamming cantrips. The monk's stunning fist has been the single most important attack in my entire arsenal, since being able to put multiple enemies out of the fight for an entire round basically at-will is unreasonably strong when everything hits you for 1/4 of your HP. But the druid's gift of life is what allowed me to complete the campaign without ever having to reload, so it's hard to say which one's my favorite.
The module has many great fights, but for me the absolute highlights were the final boss, the whole optional section in the abandoned fortress' basement, and an encounter with the orcs in the caverns where, following a lone straggler after a very tough fight, I triggered the next encounter with another huge pack of enemies. After that, I surely never followed any escaping enemy ever again.
Funny that you mention EricWisdom because he is the exact guy I alluded to earlier in this thread who is going through Forsaken Isle 6-man with just four characters.
I think most of the module would be doable with 4 characters if you're willing to expend all your resources every single fight, but I wouldn't set foot in the aforementioned basement and I think the final boss would require some very specific planning. I might try it later down the road, but playing with 6 characters has been so fun that it has probably ruined any future 4-man playthrough of any module for me.