unclebud, people are only trying to say the eMachines are a "buyer beware" type purchase. Cosnidering the price, people just want to get accross the point that you might only get what you payed for (a cheap, cheap computer). Additionally, if you check out
eMachinesSuck.com you will hear thousands of horror stories about customer service, repairs, parts failures, etc.
As far as pre-built vs. build-it-yourself that is a toss up depending on what your needs, knowledge, and skills are:
The advantages to build-it-yourself is that when something dies or needs to be replaced, generally it is not proprietary or a combination component (like a motherboard with integrated video), and you get to choose every component used in the system (translation, you don't get too many cheap/crappy parts). Thus you know what it is that needs to be replaced and how to replace that single component (I.e. if onboard video or sound fail, you might have to replace the motherboard, but if you have non-combined/proprietary components you might just replace the video card).
The disadvantage to build-it-yourself is that the tech support is harder to come buy. You can contact the manufacturers of each component, and check places on the internet (like here) but that is about it. If it turns out you bought a defective part, you might be troubleshooting for months before you realize it (depending on your skils/knowledge)
With pre-built computers the advantages are tech support, pre-configuration, pre-installation, and defective component replacement. When something breaks (assuming its underwarrantee) most companies work with you to fix it. They have their own central center for tech support rather than figuring out who made the motherboard and asking them what your problem might be.
The disadvantage to pre-built computers is that frequently they are stocked with low end components or the cheapest version of some component. Depending on your needs, cheap may not matter.
The biggest decision fact when considering any computer is need and life of the machine. If you need a machine to do work processing, balance your checkbook and surf the internet, get the cheapest thing you can that doesn't sputter along like a rust bucket. If you want to play high end 3-D games with advanced features you might want a high end machine, either from someone like Dell or a build-it-yourself (something where you have the most control over your components or the best warranty/tech support services).