Probably the best weapon system available to the general public today. Several reasons are;

High quality.

While there are a few company's out there making doo-doo for this fine weapon, the number is very small. The AR manufacturers have improved their machining tolerances and techniques to the point where we now take for granted the ability to mix and match different brand uppers and lowers knowing the fit will be excellent in the vast majority of cases. This was not the case even as late as the 90's. I for one couldnt be any happier with the great number of choices available. It is now rare to find an upper or lower of poor quality. Yes, I know. We all have our favorite(s), but be honest, you could take brand A and match it to brand B knowing it's gonna go bang when assembled correctly. This effect of high quality carries over to the accessory market as well, but there, it is even more true that you get what you pay for. Which leads to;

Accessories. 

The number of accessories is almost endless. With the giant leap in popularity of the stoner weapon family, it is now possible to set up an AR to suit practicaly anyones taste ( or lack of ). Yes. You know who "they" are. Couldnt be you or I could it? Seriously, just because someone adds items to their gun I wouldnt, so be it. The thing for me is, I can change my AR to meet any perceived need or desire. If  I want a light, fast edition to a 3 gun set-up, I can have it. Wanna 12+lb. sniper wanna be? Done. Never has an owner had so many choices. These are the golden years for AR's. And dont get me started on the price of ammo. Were all in the same boat on that, so deal with it.

Versatility.

The mouse gun can be so much more than a single use weapon. If long range precision is your game, you can do it with an AR. Varmint, National Match across the course, long range ( well, sorta ) sniper type, all of these are possible with the AR platform. No need to buy everything to change from use to use. In most cases, a change of barrel, buttstock and possibly sights, will change the whole use and functionality of your AR. Choices of caliber are also great. Anything from the 22LR up to a .50 cal thumper is possible. Break out the credit card, re-fi the house, rent out the kids. The money you "could" spend on this toy and its feeding is as limited as your means. That said, you can try to resist BRD ( Black Rifle Disease ) and keep the costs down, but best of luck to you. This is the best in adult entertainment. You will wanna get more...

Ammo;  

The day used to be you had maybe a half dozen choices in .223 or 5.56. With the proliferation of the internet, 2 dozen+ choices are now at your fingertips. Yes, it hurts to pay 300.00-500.00 per k for what used to cost half that or less ( mid 07 prices ). But at least you can still get it at a variety of places. You may have to keep an eye on your favorite seller(s) to catch when they re-stock, but it's available if you have the $$$ and patience. Concerning quality, you have many choices depending upon your needs/wants. If your gonna do some varmint hunting, they choices are good. Same goes for that Wolf  your gonna bump fire into the hillside. High dollar TAP? Gun show specials? ( yea, I know, not much "special" about those anymore ) Choices choices. Is this a great country or what? You would be hard pressed to find bad ammo even when it does come from Russia, Serbia, etc...

My experience;

I have been shooting since 1969. Yep. That makes me old school. I shot both High Power and Smallbore from 1972-1984 with several trips to Camp Perry as well as Regionals and dozens of matches in between. My father and I made High Power match rifles ( bolt guns ) as well as gunsmitthing gas guns in that time. In the last 2 years, I've gotten back into AR's. The one thing you learn after being away from something for so many years is this. There are always more ways to do something than you think. Things change. In the case of the AR-15, they changed for the better. There is always someone out there with more knowledge than you. ( Hint; this is a GOOD thing ). The way you learn is mostly from observing what others are doing and see if you can improve upon it. Don't be cocky. Be glad of what you can teach others, but not to the point of thinking you tha man/woman. Be humble and LEARN. Dont waste time trying to impress other AR enthusiest either. The number of knowledgable people out there is staggering. Take advantage. Our sport/hobby will benefit from it.

 My advice;

 If you have been on the fence concering purchasing an AR, do it. You won't be sorry. If there is a more versatile rifle out there, I dont know which it might be.  I recommend reading up on the subject before purchase. Not only as a source of knowledge, but new versions of the AR seem to arrive monthly. New toys are constantly being added to the already impressive list of accessories available. There are many fine books concering the AR. My favorite are the two written by Patrick Sweeney ( The AR-15 vol. 1 and 2 ) Their not the only, just my favorites. "The Black Rifle" 1 and 2 are tops as well. Lastly, after you have purchased your rifle, practice practice practice. Shooting becomes more enjoyable as your skills increase along with your knowledge. Check out your local and see if there are any practical, 3 gun etc.. matches scheduled and attend one or two. You dont have to be a shooter, but I'm sure you will soon want to be after talking to some of the folks and seeing what happens at a match. Ask questions. Competitive shooters are some very nice people and can help you to avoid mistakes early in the game if you listen. Shooting ( and the AR in particular ) is not cheap, but you dont have to spend money unwisely and can avoid doing so by seeing what "works" at an event. Dont worry if competition is not for you, you will still enjoy this weapon system just plinking away to your heart and wallets content. Have fun, and be safe!