We own this in my house. The first "skirmish" suggested seems very one-sided, make sure your kid, or inexperienced opponent, gets the obviously better team.
I have yet to see the expansion sets. If you love games, it's worth $39.99, otherwise I'd wait until I can find it for $10 less (as I've seen it around town a few times here). I don't agree that the paint jobs are a lot better than the D&D minis, but that's a personal thing. I think the terrain is great, and if I find a good deal on another game, I'll buy it for more terrain.
I think the game is slightly easier to learn than D&D skirmish, and there are certainly less minis to get confused over. Also, most of the pre-made scenarios have goals other than killing all of your opponents figures, which is nice.
All in all, I'd buy this again, and may buy more, even though we really play D&D skirmish here mostly (next fall is the time I begin teaching my boys D&D).