I bought 4 training pants for my DS from ebay, but almost have not used them. When I started potty train DS, it seemed like he understood those thicker training pants as nappies, and continued happily to wee and poo in there. We only moved forward with potty training when I kept his bottom either naked or loose trousers/shorts. Then he moved to normal pants.
So my genuine opinion, training pants are waste of money, but other people might think differently.