IMHO, the M17S, suitably modified by Kurt is an outstanding firearm.Basically, Kurt does the final development/improvements which Bushmaster was unwilling to do.
In fact it has some significant advantages when compared with the AR-15 series of rifles/carbines.
Properly modified, I would prefer to field with an M17S rather than an AR.
YMMV.

From xanadu:
"Has almost no parts interchangeability with anything other than another M17..."

I've yet to see anything break or wear out on an M17S.  The overall number of parts is far fewer than on most ARs, and the design is inherently robust.  However, the critical parts in any semi-auto rifle are its extractor, ejector, and their springs.  Fortunately, these parts are straight from the AR series of rifles, along with some others as well.  Bushmaster sells a field re-build kit for the M17S which will probably set the average user up for life.  Price, IIRC, is about $35.
Mags, of course, are straight AR.

Some reasons for considering the M17S are:

Unlike the AR, it does not sh!t where it eats.  Reliability is enhanced, and maintenence requirements are greatly eased.
In its stock configuration it has a barrel length of 21 and 1/2 inches, giving increased range over any AR, especially M4gerys.  And yet, even so, the overall length is less than most M4-style carbines.

xanadu is right when he says that the M17S will not retain its value as well as most ARs.  However, that fact is irrelevent to the inherent qualities of the rifles, and has more to do with ignorance of the positive qualities of the M17S.

From BushMeister:

"Bullpups are uglier than Madeline Allbright"

To paraphrase Forrest Gump, "Beauty is as beauty does".  And a properly modified M17s will do quite well indeed.