This is not a game; it is a simulator in the purest sense. Unlike Euro Truck Simulator 2 and similar, there is no aspect of gameplay at all.
As such, if you don't really, really want to drive a bus, then I can't reccommend OMSI for you.
On the other hand, if, like me, buses are your thing, then this is the best - no two ways about it. The driving physics are excellent, the sounds are just superb, every individual switch in the cab works and attention to detail has been layered upon the vehicles you'll drive. The defualt maps are OK, and the default buses are excellent - but this simulator is all about the mods. There are dozens upon dozens of amazing freeware buses and maps out there, and a few excellent officially endorsed payware items (I highly reccomend the O305 and Three Generations bus packs).
In case none of the maps available meet your taste, the game includes a map editor which allows you absolute freedom to build your own maps, even if the controls take a little getting used to and the editor itself is really well hidden (google how to run it). With a little patience, you can learn to create assets for the game without too much difficulty in Blender and Sketchup, or, in my case, Gmax, via a very circuitous import route.
There are limitations - the game only really supports right-hand traffic, for instance. There are left-traffic routes and buses available, but there are issues with passengers walking through the bus. You'd also better get used to selling tickets in German.
There are flaws as well - you need a fast processor to run this game at all (multiple cores don't help), and an SSD, while not absolutely necessary, will significantly boost performance, as the game is constantly loading items as they come within range. Back when it was released, the game was not stable at all. This seems to be fixed for most people now, but you should be aware of it nonetheless. The physics can be jumpy occasionally, and I've had a few weird bugs like the back end of an articulated bus falling off. This kind of thing has been few and far between for me, though the game becomes less stable the more mods you have (clean up the ones you aren't using). Play with the settings until you get something that seems to be stable.
All in all, if you like buses, this is simply a must-have. Yes, there are bugs, but they're outweighed by the sounds, the driving physics, the wholly interactive dashboards and the endless customization.