With that ominous note hanging in the air let me say Lagoon is...
...
...
...quite possibly the best trackmania environment Nadeo has ever released. I have not really had time to decide if it beats out Canyon as an environment in my mind but even at worst, since it combines the best parts of the island and bay environments from trackmania sunrise with the wonderful offroad driving from valley and rollercoasters it's definitely up there with Canyon. And Lagoon is the best title release from Nadeo, in terms of polish, features and campaign.
A few things I particularly love in no order at all:
Rollercoasters! Yes we had these in Turbo but it's great to finally see them in Trackmania squared. And not just in Lagoon, which surprised me.
Wonderfully inventive campaign tracks. Even when they're not playing with magnets they force you to think in new and unusual ways (D14 is a really good example). This is trackmania at its best and its only the official campaign.
One of the best trackmania songs in a long time. Because while the soundtrack on the whole may not be quite as good as Valley's, "Yenset" may just be a better song than "Fill Your Pages", which has been the undisputed champion of trackmania songs for over a decade. And even if it falls slightly short of Valley the soundtrack is still better than Stadium and miles ahead of Canyon.
...come to think of it, the one thing that would make Yenset better would be pitch-shifted unintelligible voices singing about death. Imagine that...
It's been moved again. Nowhere near as far as Valley was but while Turbo Lagoon seemed pretty clearly to be in Sri Lanka this version looks more likely to be in India or maybe Thailand.
The day-night cycle -- yes every environment has this now but Lagoon looks like it's built for it. Buildings cast unusual shadows at odd angles, water sparkles in a myriad different ways and the sky always seems to be doing something interesting. The night especially reminds me of travelling to Central America when I was 14... even though Lagoon's in Asia.

The car's handling seems slightly different, but in a good way. The beach driving in particular feels a lot easier than it did in Turbo but that may be just because I wound up practicing it so much in Turbo.
Now this isn't to say everything in Lagoon is perfect. There are a few small negatives to point out here too:
The waterline on the beaches is noticeably jagged. Compared to the beautifully realistic look of everything else here this, and the somewhat lackluster texture under the grass after you drive over it, really stick out.
The texture under the grass. If it's going to look like that I'd honestly prefer if it if the car wasn't also a lawnmower at all.
The campaign, apart from a couple of difficulty spikes, is really easy compared to Valley, Stadium, or especially Canyon. For example on Valley it took me months to win gold on the first tier red tracks. On Lagoon I had those five gold medals within a day. I don't know how much of this is due to me already having some experience with the car's handling from Turbo, but even so, I won more Nadeo medals on Lagoon in a day than I think I ever had on Canyon, Valley, and Stadium combined.
The entire campaign is still really really good, but those difficulty spikes (D14 comes to mind again) are where it shines the most.
And this is more of a nitpick than anything else, but I absolutely loved the soundtrack to Trackmania Turbo. It would have been cool to have one or two of those songs appear in their "full length" alongside Professor Kliq's music. Especially since a few of the best songs from Turbo don't actually seem to exist anymore -- I've looked long and hard, and so have other people; but they're out of print and don't seem to exist online for purchase or even streaming in any form either. But presumably somebody at Nadeo has them on vinyl, tape or CD since they were able to use them.

And since I'm calling this a review I might as well give it a rating too.

Overall Lagoon is a 9 out of 10. It was good in Trackmania Turbo, but the Maniaplanet version is what makes it truly great. Definitely worth the extra $$.