Their as good as any other brand from what I've seen over the last 4 years that I've bought them,can't vouch for their customer service as I've not needed it in that time with 6 personal purchases going on 7 with the Vostro I just ordered.
I would highly doubt you will be playing any DX10 games at medium settings in DX10 mode and I really don't think any of the current mobile 8 series will looking at the framerates and settings for the desktop cards in the few DX10 games there are.
I don't really understand peoples obsession with DX10 as it is very new and has obvious performance penalties that will require much lower settings,which I imagine will negate the reason for using DX10 in the first place.Crysis or some of the games yet to be released might be better in this regard,but I can still see developers putting more effort into DX9 because thats where the largest user base is and will be for a little while yet.
As far as being fast,that would depend on how its configured,if your talking purely games,it should be middle of the road for the midrange type laptop it is,the Asus G1S certainly appears faster as far as gaming benchmarks goes,but it also costs 50% more where I live,so bang for buck,dell seems to have done pretty well IMO.