At the risk of being behind the technology power curve, i ask the question:  what is the advantages of the SIR systems and how do you like yours?

Free float the barrel, best cooling and heat dissipation capability on the market, no uncomfortable rails to hold on to, yet enough rails to mount whatever I want where ever I want. Ultra solid and rigid top rail to mount whatever optic I want where ever I want. Synthetic lower half which does not get hot, period. One of the easiest free float systems to install, or take off. Bottom half can be removed for barrel maintenence without the need to mess with the upper half. The list goes on and on. I love them.

What he said.

new-arguy pretty much summed it up.  A couple other benefits, on the SIR-50 you can co-witness an Eotech and place the Eo out over the barrel.  You have flexibilty as to where you want your rails to be and not to be.  Also, the system comes standard with a side sling swivel, an extra cost with some of the other systems I've seen.

I like mine quite a bit.  When the #50 was announced, I had MSTN get me one as soon as they had them in stock.  I've used a RAS equipped rifles, various Fobus setups and some free float tube setups and the SIR system fit the bill for me better than these other systems.  I would guess that the FF RAS would be on par with the #45 SIR.  

While they are not cheap, they do have more benefits for the buck than other companies similar setups in my opinion.  If you set a rifle up with one, I dont think you'll be disappointed.  Good Luck!  

Burnsome:  I like being behind the curve.  You get good info from early-adopters, don't risk untried products, and can watch how they hold-up over time.

As someone shopping for a FF system, not a user (yet) I have to say the SIR's major attraction is the ease of retrofitting to assembled uppers.

New-ARguy hit the nail on the head.

The only disadvantage is that when fitted with the SIR and Magpul stock you get those "But it look's like Robocop's leg!" comments that are seen on the board so very often [beer]

-Cap'n

Another thing usually over looked is those threaded holes along the sides of the SIR's, they go to a wiring harness that gets installed to alow battery operated optic's to plug in to a power source. No more cables in the not too distant future. That's one of the other benifits of the extra cooling ability.
The SIR is far out in front for the future  for high tech in the works.
Good shootin, Jack

I absolutely love my SIR 46 except that if I attach anything it will stand out more than most of the other rail handguards. But another plus is that if you are not attaching anything the 6 o'clock you don't need to put any rubber protectors there since SIR's lower is synthetic . While I think it is very good for using without a vertical foregrip, when you use a regular foregrip and attach flashlights and stuff, it tends to come out more and sometimes makes operation of these accesorries harder. Also it is a bit heavier, and one can see that the sleeve is unnecessarily long (on the other hand thats the reason why it is famous for solid uninterrupted rail)

I'm just throwing some stuff I noticed after using it a bit. But I do like mine? Yes of course. Since all SIRs are user-installable it is great too. Now all I need to AN/PEQ-2 to go on top of my SIR [:D]

I got my 50 a few weeks back... threw it on in about 10 minutes, and went shooting after work. And then pretty much every day after that.

I love mine, the big problem i've found is that once you get it, you start pricing out all the other crap you can hang from it, and than you realize you are going to need to find a second job.