Its a early semi-automatic rifle, and is about average for reliability for a early semi-automatic rifle. It is not a SKS, nor a AR-15, nor a M1 Garand, it is not going to keep functioning no matter what. Just don't expect it to be this amazing works 100% of the time rifle which is super accurate and reliable. There is a reason it didn't last long in Russian service.
Issues you might run into:
Ammo: the SVT-40 is a picky gun, some guns work with any type of ammo, others (like mine) are very specific with what types of ammo they like. I have to buy modern ammo as most surplus I have used hasn't worked properly (i.e. click, hard primer strike, try another round, bang).
Disassembly: make sure it is clean before you shoot it, and make sure you know how to disassemble it. It is a real pain to do so despite what others claim. By far the most difficult rifle to disassemble which I have ever owned (and I have owned quite a few including many early semi-autos), but that being said it isn't too bad once you get the hang of it. I would recommend shooting modern ammo out of it (MFS was .50 cents a round last I checked and it worked great out of my SVT-40) as it is non-corrosive so you don't have to worry about that so much (and have to disassemble the rifle again in a couple days to verify the condition of it).
Accuracy: They are not the most accurate rifles out there, you will hit the target at 100m, just don't expect 1 inch groups.
Overall they are cool rifles which have a awesome look to them, they can be a lot of fun or they can be super frustrating (which was how mine was when I found it didn't like most surplus). Just don't expect too much out of it and you will be fine.