Speaking as someone who is learning to fly NDB approaches at the moment, I think they are an excellent training aid and without doubt increase your piloting skills. If you can fly an NDB approach accurately, you'll fly a VOR or VOR/DME without any problems.
In the real world though, I would hate to have to use an NDB to land in weather close to approach minima. Perhaps it is just in the PA28.. but I find that even with the needle sitting right where it should be, you tend to end up significantly off the centre line. The ones around my part of the world really deserve the title "non-precision"!!