I've bought for myself about 10 Dells over the years and my company has bought probably about 200 of them over the years. They have always worked fine for everything, including gaming for me (Aces High, Battlefield 1942, Quake, WWIIOL, etc.).
These days, it costs $480 for an Inspiron with a Core 2 Quad Q8200, 2 GB RAM, 320 GB hard disk, 16x DVD+/-R/W drive. The power supply in them is sufficient to run a $100-$150 graphics card (I've done that on my last several Dells), which is more than powerful enough for all games I'd be running.
Could you build that system for less than $480 on your own? Probably, but I think it would be hard to knock all that much off the price, and it would take a bunch of your hours to shop and assemble.
Every few years I just buy another low-end Dell with a mid-range (or higher) CPU and put a $100 graphics card into it. Nonstandard case and power supply don't matter because my upgrade path is buying an inexpensive system every few years. Dell does install a bunch of crapware, but I either uninstall it or reinstall the OS.
However, for folks who like to build systems from components as a hobby or to keep technical skills up to date, or if they want a system with much-higher performance than matters in Aces High, those are all excellent reasons to go the do-it-yourself route.