I've made good and bad wines with Alexander's. On the whole, I think their white concentrates are a lot better than the reds, maybe because they tolerate the concentration a bit better.
I made a passion fruit riesling with Alexander's Riesling and 64 oz of passion fruit syrup, and it's very good. I've been told by more than one person that they would buy it if I sold it.
The key is no more than a 3 gallon batch with one can, and even then mix in some fruit or other juice to help out. I've used 2 cans for a 4 gallon batch and it turned out pretty good.
As far as the reds go, I have only had decent results with the Barbera. I mix it with cherries or cherry juice and make Cherry Barbera.
In most cases you probably would get better wine from a kit, but better than using frozen welches. The cool thing about Alexander's is that it's cheap enough to experiment with, and it's a good way to learn things like acid adjustment, hydrometer reading, clearing, etc without chancing messing up an expensive kit.