I rented a Barrett M99 a couple of years back at a range near San Antonio. With the rifle we got 10 rounds between the two of us there. We asked for more but the guy behind the counter gave us a look before saying 'See how you get on with those first then come back if you need more'.They were comfortable to fire, and surprisingly accurate for a semi auto.
It was possibly the most horrible rifle I have ever fired. Breech loaded and with no mechanical recoil supression it was nasty to say the least. By the time we had taken three shots each we were looking at each other in the vain hope the other would want to use up the remaining rounds.
A definate 'tick' on the list of weapon types fired, but not one I would like to do again. And the sheer weight of the thing (+ ammo) makes you realise just why they are not widely adopted as a 'field' weapon. Heavy, low rate of fire, limited use beyond a few specific purposes (which can often be done by other more portable systems) and tying down manpower who could otherwise be better employed on the battlefield. Too many negatives and not enough positives I think.
I wouldn't be surprised if they have a few in various SF armouries gathering dust too.