The single-player modes have flaws, but the multiplayer and online more than make up for it.
It's also difficult to sum up the whole experience up in just a few short paragraphs, because the game runs so incredibly deep in both its online and offline modes. There's so much to do between vehicle customization, races, platforming, time trials, and building tracks, you will literally sink 40-50 hours into this game and barely scratch the surface of what it can actually do. The thing is that regardless of how you play it, or what you play it for, you'll find some serious value here.
All of this fun comes with a word of caution, though. When you buy this game, buy it for the online and customization options. Do not buy it for the single-player experience. Sure, there is literally tons of solo content to be played here (the time trials alone have 147 available tracks), but the frustrations you'll endure trying to unlock all of it isn't worth the effort.
Because the game is controlled with the arrow keys on your keyboard, the steering is already going to be somewhat imprecise and present a slight challenge on its own, but when Nadeo starts throwing in things like pinched off corners, tiny spaces to drive through where your vehicle will barely fit, unforgiving high-speed jumps, and obstacles that you virtually can't see at high speeds placed right where you would normally drive, it's just insane. Your frustration will run high enough on some tracks to make you put your head through your monitor.
But once you take your head out of the grinder and move away from that crap, you'll find there's a whole of bevy of customization options waiting to be explored. You can build your own tracks, paint your own vehicles any way you want, and even change your horn to any .wav file you can get your hands on.
Once you go online it gets even better. Trackmania uses an in-game currency system called "Coppers" that you earn by accomplishing various tasks (i.e. beating a certain time on a given track), and you can use these to purchase add-on content such as new tracks, paint jobs, sounds, and car models which range from everything from the ordinary to the extraordinary. If you're willing to spend the coppers, you can race around in Darth Vader's Tie Fighter, for example, complete with a laser-cannon horn.
Online is also where all of the competition is. Most of the servers I found had plenty of other players to race against, all of us trying to beat each other out for the best time possible for 6 - 7 minutes before switching to the next track. You'll find some servers with over-complicated or incomplete tracks, but for the most part it's a lot of fun. Not to mention it's a great feeling when you beat the #1 position by 5 whole seconds or more.
My final recommendations on this one are mixed. Solo players need not apply here, unless you like a little masochism with your gameplay. The single-player experience grows stale and frustrating after a few hours, and doesn't really reward you for your work. Finally beating one frustrating track just to open another, even more frustrating track doesn't cut it.
However, if you love a good arcade racer, really enjoy making things look exactly the way you want them to, and have a creative bone or two in your body you're going to love it, and it's absolutely worth every penny. There's countless hours of fun to be had here for both modders and competitive players, and that goes double if you're both.