As mentioned above, Dell doesn't use quality hardware, and sometimes not even standard hardware (they recently used a standard ATX power connector with swapped pins, so you can't use a standard ATX power supply.)
The reality is that any brand name system is just a bottom-dollar build, with cheap components... and for that matter, it doesn't matter if you buy Dell, Gateway, HP, eMachines, or whatever. Now, if you pay to upgrade, then you are certainly improving things, but often enough, the higher prices systems are a ripoff when it comes to price vs. performance.
Lastly, I hear it all the time: "I bought <insert brand name> because I wanted the support in case something goes wrong." Well, I hate to tell you, but ALL of these companies have crappy support. Their common solution to any software issue is to reformat and reinstall windows. I have had many customers come to me with "broken" systems but they were wise enough NOT to reformat, as it would lose their data. You don't have to reformat to reinstall windows... and more importantly, if you are competent, 90% of the time you don't even need to reinstall windows. Reinstallation is just a cop-out. Granted, from a business perspective, it makes more sense to them... but that's not SERVICE... that's just a restore CD or a Ghost image.
I could go on and on about terrible support from Dell and other companies, but I won't bother. It's a dead horse. A home-built system will certainly outperform it, sometimes for less money, too.
As for the every-day user who just want's email and such, sure, a bottom-line system is plenty for them. Again, it doesn't matter who it comes from, it's still crap. However, just don't get caught up in trying to upgrade it too much.
Heck, with that mentality, most websurfers don't need a CPU faster than 1GHz, or more than 256MB RAM... yet they buy faster and faster machines (mostly because the Dells of the world don't make slower boxes anymore.)