I agree, the inevitable friction will slow it down, but it should run for a minute or so ... long enough to be satisfying.

If it's gimmicky, then it's likely that not so much care would have been made to make sure it works properly, so the forces would be unbalanced and it would stop relatively quickly. As you've said, the strings are bent and the alignment isn't perfect, so every time one of the balls hits another some of the energy is being wasted. The size of it matters as well though. If you have two Newton's Cradles that are made in exactly the same way but one is bigger, then the bigger one will go for longer.

With a big one, you lift up the first ball higher than you would for the smaller one and so you give it more gravitational energy, which means that it will take longer for the energy to be dissipated by the friction, because there's more of it. Also, the bigger one would have heavier balls so the air resistance would have less effect on it. Think falling hammer and feather.

I've got one that's currently tangled up (if you're buying it for a child you'll need to watch out for that), but before it went into the unfixable mess that it's in now, it could last for about a minute before it slowed down significantly.