Any thoughts from current owners?
Glad to hear you are considering the D-15.  I am a Martin fan so I may be biased here so take that into consideration.
Many say that the D-15 is the best all solid wood guitar you can get from Martin for the money.  I agree 100%.  I have owned mine now for about 4 months and we are doing great together.  I bought it as a substitute for my other Martin while it is in the repair shop for the lifting bridge.  
I bought mine from maurysmusic.com for $599.  This price included a free bone upgrade, setup and case which all Martin guitars come with.  The owner, Maury, is a great guy to deal with and does a solid shipping job.  To get the free bone upgrade you have to be a member to umgf.com.  I had my action lowered and I am glad I did.  Play smooth and fast.  It is a bassy guitar, when compared to others, but it still blends well with the treble to produce a nice Martin sound.
As for strings I use mediums on it.  I like a loud guitar and I personally find that light and extra light stings dont do it for me.  I have been using D'Addario EJ17's.  Not saying their the best but I like them, expecially when I can get a set for around $5.50.  I am sure though that lights would work on the D-15 so give them a try if you want to go lighter.
Finally, I will say that the D-15 is not an incredibly bright guitar when compared to solid spruce top guitars.  It has more of a mellow sound.  It does still have the capibility to do all that you want it to.  Try a few out.
Let me know if I can answer any other questions.