Any reason to have a dedicated rifle, rather than the conversion kit?  I am thinking of the DPMS.  Any comments?

I recommend the DPMS dedicated rimfire upper.
It is built very well.
Most importantly, dealing with DPMS customer service has been a very good experience.
Dunno if you'll get that with the Ciener conversion.

The DPMS will have better resale value as well.

Here's a pic of my Kuehl dedicated upper.  Click here for his website:  kuehlprecisionfirearms.com/  Dedicated uppers have the proper bore size and the  1/16 barrel twist which gives better accuracy.  With a dedicated upper, I am not concerned with clogging the gas port.  In the case of the Kuehl upper, the chamber adapter is omitted and the barrel can be set further into the upper receiver.   This gives it a more authentic looking M4 barrel while maintaining 16 inches in length.  Kuehl uses a modified Ciener conversion kit and it takes 30 round mags (currently, very spendy at around $120 each).  Kuehl uppers are not inexpensive, but they are of high quality and Rick is great to deal with.   I've read good things about the DPMS upper, but have no personal experience with them.  

Dan

Dedicated uppers have the proper bore size and the 1/16 barrel twist which gives better accuracy.

This is true with the DPMS upper also isn't it?

Quoted: Dedicated uppers have the proper bore size and the 1/16 barrel twist which gives better accuracy. This is true with the DPMS upper also isn't it?

Yes.