Simply put, the best reason to build your own AR15 is so that it can meet your exact specifications and so that you can customize it and make it your own.
I like that it gives me the ability to know and understand the design and function of the AR15 rifle. This gives me a better idea on how to diagnose potential problems and fix them.
It also gives me the ability to hand inspect every part in the weapon to determine quality (to some degree). I can swap out and upgrade parts as well as refinish them in order to improve quality of the part and the weapon as a whole.
It's also nice to be able to make changes down the road when I decide that I want a different upper style, barrel profile/length, etc...
There are various degrees of AR15 building. Anywhere from buying two separate halves (a complete upper and a complete lower) and mating them together (the easiest way to "build") to finishing out an 80% stripped lower and turning down a barrel on a lathe (something that takes a lot more time, tools, and savvy).
I read an article not too long ago about the downsides of what some people refer to as "frankenrifles", home built, or even kit AR15's. It went on to mention that these rifles were not reliable and should not be trusted for personal defense.
A custom built 14.5" AR15 carbine (with a permanently attached 1.5" Phantom Flash Hider), Hogue grip, and Daniel Defense rail
While I agree that many of the cheaper "kit rifles" may be lacking the quality, a home build very well can give a savvy AR15 builder a better rifle than can be purchased off the shelf. However, this starts with using quality components from reputable companies, especially where it matters most.
A custom built 14.5" AR15 carbine (with a permanently attached 1.5" Phantom Flash Hider), Hogue grip, CTR stock, LaRue rail, and ACOG (light sabre not included)
Many people often think that you can save money on AR15's by building them, this can be true but it tends to be most true when you desire to build the best possible rifle for the money. This is because you can buy many parts such as a forged anodized lower receiver from many reputable companies for much cheaper than if you went straight to LMT or Noveseke. Most AR15 lower receivers are going to be the same in quality, regardless of manufacturer. The same is true with many other parts.
The parts that are the most crucial to get from high quality manufacturers tend to be barrels and bolts foremost, with special consideration given to other parts such as triggers, sights, buffer tubes, stocks, etc...
Customized AR15 A2 flash hider was turned down on the ends with a lathe and given a more unique look to better fit the skinny profile of this 16" pencil barrel AR15 carbine
Depending on the degree of customization you will need a variety of tools. To simply assemble an AR15 lower to a complete lower, you would need the stripped lower, a lower parts kit, desired sling mounts, and a buttstock assembly. The tools needed would be a set of pin punches, a small hammer, stock wrench (for adjustable carbine stocks), allen wrench, loctite, and possibly a flat head screwdriver.
To build an upper, tools needed would need a barrel wrench or AR15 armorers tool, action blocks for the upper receiver, vise (to hold the action bolcks), pin punches, etc...
This along with the barrel (including front sight base/gas block, gas tube, handguards, and possibly barrel nut, delta ring, etc...), upper receiver, upper reciever parts kit, sights and/or optic of choice, desired sling mounts, among other possible things.
And to a larger degree, depending on the complexity of your build, you may want or need an air compressor, drill press, silver solder, lathe, etc...
I have personally helped dozens of friends and family members with numerous builds in a variety of different configurations.
Compatibility between various brands usually is a non-issue. Once in a while, you will get an upper from brand "X" and a lower from brand "Y" that will have a tighter and looser fit. A tight fit is usually cured by usage of the AR15, shooting, assembling and disassembling. A loose fit can be cured with an Accu-Wedge.
Sometimes the finishes will not match perfectly (this is merely cosmetic and will not affect the function at all). Receivers can have anywhere from a black, gray, or even purplish coloring depending on the manufacturer and the particular “batch”. Refinishing is an option if this bothers you that much.
Of course, there is also the option of swapping out the receivers.
Fit and finish is nice, but it doesn't translate to anything where the rubber meets the road.
Many of Colts newer AR-15’s are made with larger pin hole sizes that won’t fit the majority of receivers out there (except the ones they made to match them) as well as odd (large) sized FCG's in an effort to over-comply and appease the ATF. This way, their “civilian” AR-15’s won’t be compatible with M16’s. Normal Receivers and lower parts kits won’t be compatible with these Colt receivers either, as Colt is the only one to have taken this drastic measure. It’s annoying and it keeps me from ever wanting many parts from Colt in my AR-15’s. Many people feel that Colt has sold out to the ATF and boycott their products altogether (I haven't gone that far yet).
You can match up Colt lower receivers with other types of uppers if you get an adapter for the large pivot pin.
Having built several AR-15’s, I can say that there is very little problem using parts from different manufacturers to construct an AR-15. I have rarely assembled an AR-15 with all the parts being from one particular manufacturer. All of my personal AR-15’s are mutts.
A few things you should know about building an AR15
Federal law Prohibits you from building a rifle with a total barrel length of less than 16" unless you apply for and are granted the $200 tax stamp for an SBR (short barreled rifle).
If you do not have the tax stamp and intend to put the upper on a rifle lower that is not registered with the ATF, you must be sure that your barrel is the minimum required length. In order to achieve this minimum length on a shorter barrel (than 16") such as a 14.5" barrel, you need to permanently attach a muzzle device that will bring the overall barrel length to 16" or greater.
If you are building an AR15 pistol, you need to register it as a pistol from the moment you get it from the FFL. A pistol AR15 CANNOT have a buttstock or a vertical foregrip.
A registered SBR can have a buttstock and a barrel of any length.
Although I oppose many of the laws regarding restriction on barrel lengths, select fire capabilities, etc... I do NOT recommend or condone ANY illegal modification to your AR15. It is simply not worth it.
Disclaimer: Some states, cities, and counties make it illegal to own certain configurations including but not limited to SBR's and NFA. Check your local laws to be sure that you are in compliance. I am not a lawyer, nor should I be considered an offical source on laws in regards to AR15's. I do not accept responsibility for any illegal actions taken by the reader through misunderstanding of my writings, the readers lack of research, or otherwise.