I tried one, a police SBR with iron sights, last year.
It was bought because the department had adopted the patrol carbine concept but their M4 didn't fit on their motorcycles. First impressions were good. It was very light and it balanced beautifully, even with a fully-loaded Beta-mag. Being brand-new, before taking it to the range I'd cleaned and lubed it.
We fired about 500 rounds with no stoppage and no issues (except for rather large groups). I would have preferred to have a quality red-dot optic on it, but, nonetheless, I came away impressed.
Besides the optic, a hand stop would be a good idea, in my opinion, but they did not fit the motorcycle rack. Another attachment that might make sense would be a glass-breaker mounted toward the muzzle.
Since then, I've been told that feedback (from the motorcycle patrol officers') has been very positive.
P.S.: It was 5.56 and had the the handguard from the below model.