DISCLAIMER: I'm not an engineer, I just play one on Woot.
I bought my DC25 from Woot back in 2010. I loved it. The honeymoon was fantastic! But now the honeymoon is over. I still love the Dyson, but the infatuation is over as now I see some of it's flaw. And I've been tempted to cheat with another brand, or at least with one of it's siblings such as the DC28.
Here's some of my gripes.
1) The self adjusting, i.e. "floating" head, doesn't do a great job on shorter pile carpeting, especially carpeting that has some years on it. Instead of actually getting into the nap, it bounces over it which leaves debris behind. (I have a minor workaround for this, but that's not the point. You shouldn't need a workaround.)
2) The suction occurs in the middle of the head. But the helix brush goes all the way along the roller. What this means is that half of the brush is pushing dirt towards the suction point. The other half of the brush is pushing dirt away from the suction point.
In order to achieve optimal cleaning, you need to always work from one side to other. (I don't recall if it's left to right or right to left.) Also, for sure when you are vacuuming along the baseboards/walls, you definitely have to have the the right-side of the vacuum along the wall, which means you'd be going counter-clockwise around your house. Otherwise the brush head is pushing dirt away from the suction point and back towards the wall.
3) The wire that loops around the hose is so tight that it barely stretches more than a foot or two before it pulls the vacuum, which normally results in it tipping over. It's not very good for getting the full extension from the hose for doing steps or up in corners/ceilings.
4) This is more minor, but since it has the 'floating' head versus a choice of fixed positions, things can get nasty on a long pile carpet, like shag or if you have a rug comprised of microfiber. In this case, it sucks up too much of the fabric and can potential lock up the roller.
All that said, I still really like the vacuum. The roller ball definitely makes it easy to handle and go around corners, but the other side of that is that it can be difficult to maintain straight lines if you're one of those OCD types that require your vacuum lines to be 100% parallel.
Since the primary selling point of the Dysons used to be the "cyclone" technology, without having other "cyclone"-type vacuums to try, I can't say that this one is the best anymore. Other mfgs such as Hoover and Eureka may have comparable offerings for significantly less.