I say GET IT!!! (If the price is good).
I consider the M17S [b]SUPERIOR[/b] to the AR for several reasons.
(1) 21" barrel in 30" rifle. You get a rifle shorter than an AR 16" carbine, but with a barrel longer than a AR 20" rifle. Great accuracy and huge muzzle velocity.
(2) Balance is FAR better. You can shoot the M17S one handed all day if you have to. It is UNTRUE that it is heavy, btw. Since the barrel is the heavist component of a rifle, at the very least the M17S with a 21" barrel should be compared to an AR 20" rifle - and they weight pretty much exactly the same.
(3) Far superior gas system. The M17S uses a piston, so unlike the AR it doesn't blast dirty gas into the mechanism, fouling everything. It is a more reliabale design, and it requires far less cleaning.
(4) I actually like the big long single-piece receiver, because it seems that much more sturdy and rugged than my AR, which seems flismy and fragile by comparison.
The downsides are that
(A) the existing handle (w. the iron sights) sucks, and you would want to either mount a rail directly on the received (w. Kurt's folding charging handle behind it), or do something else, and you'll need optics.
and (B) the trigger is not great. For a military-style rifle, it is not bad, and with a little bit of tweaking and polishing, it can be easily as good as a stock AR.
However, since the M17S costs at least $200 less than a comparable AR (and even less if you look around), I think it is totally worth it, since that can easily pay for the modifications, and with minor tweaks, it is a far better rifle than the AR.
I have an AR and a modified M17S, and if I were to buy another rifle, it would without question be an M17S.