oops, yes forgot about aquamist. Kevin can probably also speak of the advantages of buying a pre-made kit as well. Plus they look good too in kevins case..
With a custom kit, you dont really get a "controller" certain kits will alow you to change the meth turn on point with a adjustment of a dial in your dash or center console or whatever, and also you can adjust how fast the meth will hit its full pressure as well. They also have LED lights built into the controller that show you when the meth turns on, if the meth level in your tank is getting low, and can also indicate trouble "codes" requarding the system if it detects excessive or low voltages going to the pump. All of this does not come with a "build it yourself" kit. Certain kits will also work with a electric boost controller, or ECU, to lower your boost and timing, if it detects a problem with the meth system. Again, you dont get this with a "build it yourself" kit.. and it can end up saving a engine in some cases, which can costs thousands and thousands of of dollars and time.
Me myself, being a experienced journeyman mechanic, can definalty see the advantages of a aftermarket kit"... plus your not trying to source all of the pieces sepratly, and then find out your missing some components, plus all the time you spend driving around to gather all the new pieces... and then find out that some of the pieces dont work correctly with other pieces you purchaced, and then you end up with a system that works... but it doesnt have all the advantages of the progressive controlled kits with built in warnings and options.....
So are you really ahead by saving a 100 or 200$ in the end??? not im my books. but if that $100 or $200 is life or death for you.., and you dont mind wasting time and effort putting the kit together, plus trying to figure out how to install it, and you just want a simple setup, then i guess building your own meth kit would be worth it