I use my miles for domestic R/T based upon the price of the trip. With the exception of Southwest, the cost of the R/T must exceed $300.
Due to timing, I have sometimes ponied up with the max miles just in order to get the flights. Like last summer I got a R/T to COS for my "friend" for 40K United miles, rather than a $1,500 last minute fare. A good use of miles in my opinion.