(REVISED) Some issues with the "sim"-ness of the game aside, this is one of the most satisfying racing games I've played. That's largely in part to the huge mod content available.
Compared to the hundreds of cars that games like Gran Turismo and Forza have ballooned into, even the modding community can't offer a comparable volume on that front, but it makes up for that in the quality of many of the car liveries (or "skins") and sheer volume of race tracks. You won't find a console racing game or many PC ones that offer the volume of race tracks you can download for GT Legends (and presumably its sister games, too). At the moment, I have more than 100 tracks installed, and there are lots more available. Not only can you find different tracks from around the world, and a few made-up ones, but some classic tracks -- such as Le Mans, Spa, Monaco, Nurburgring, and others -- offer some of their various incarnations over the years. In short, I've never seen a racing game offer more opportunites to race the tracks you've always wanted to race.
The visual quality of the tracks varies, some looking plain and dated, with textures like roads and trees looking painful old in some, but pleasantly realistic in others. This is also true for the creativity of the "fantasy" tracks, some a little bland, but others achieve what they advertise very well, like the "Moorlands" track looking like it came right out of Wuthering Heights.
Besides the mods, the game offers suprisingly good-looking cars and still some of the most impressive natural-light effects, particularly at dawn and dusk and how the light looks on the cars.
The "sim" aspect is also mixed. Having never raced real cars (much less any like these old ones), I can only compare the sim authenticity to my instincts and the closest game equivalent I've played -- namely, Forza. It feels a little overcooked in parts, with certain physics seeming oversensitive, like the grass and sand feeling more like ice; some cars getting lost in a slide that renders counter (or any) steering ineffective; cars flip too easily and overreact to some contact; some cars' inability to hold a straignt line under braking. Not only do they feel overdone, but vindictively so, too often pushing it beyond challenge into unbearable irritation. In short, while some of the over/understeer and other physics are impressive and satisfying, there seem to be just as many elements that make this as aggravating as often as it can be fun.
Perhaps some of the over-fidgityness of the cars are avoided by using a racing wheel instead of a controller, like I'm using (PS3), which also doesn't recognize any rumble/feedback.
Also vexing, on the single player side is the poor balancing of AI and grid competition, and the relative uselessness of many cars in a mixed grid. That means that the number of actually usable cars -- in terms of being able to win the races on the harder difficulties -- is limited. You are stuck with whatever car you pick for all the races in a particular "tournament", so there are really only two or three choices (at most!) to give yourself a realistic chance of winning on pro or semi-pro. I imagine you can find (or create) online one-make races, though, that would allow you to drive all these nice classic rides.
Ultimately, the well-aged visuals, the mixed but overall still adequate sim-racing experience, and the gobs of mod content (most notably, the tracks!) makes this a must-have for serious racing game fans.