I bought one of these new on Dyson's Cyber Monday sale last year. I paid about $375 plus tax for it as a new model (the Dyson-wide sale was 25% off retail).
So far it's been a great vac. As far as suction goes, it's strong and forms a good seal with my short pile carpet. Judging by the amount of dust and cat hair I empty out of the bin each time, it's getting the dirt that ground in pretty deep. On my hard floors, it just sucks up everything there. It works great.
According to Dyson's published stats, the DC39 has the highest suction (as measured in AirWatts - whatever the heck that is) of any Dyson vac.
As other posters have mentioned, the handle and wand assembly is very light and easy to maneuver. It can easily fit underneath most furniture given a 3"-4" clearance, so you'll be able to vacuum areas that you'd otherwise have to move furniture if you wanted to get with an upright.
The canister maneuvers pretty easily, though you do have to give it a firm tug. I also love that the power cord is mounted on a spring coil and retracts into the motor unit with the press of a button.
The filter feels to my nose like it works very well. Vacuums have to move a lot of air through them, and the air vented out into the room has no noticeable odor or dust to aggravate my allergies.
The two biggest downsides to this model are storing the unit in between uses, and emptying the dustbin.
The telescoping wand has a tab on it that's supposed to fit into a notch on the motor unit. It does, but it takes some work lining everything up, and it feels to me like it's likely to break at some point down the road. Also, with the wand hooked onto the motor unit, the hose doesn't really have any way to sit cleanly. It just kind of sticks out in front of the machine. Thankfully everything connects and disconnects easily, so the most compact way to store the vac is to disconnect the hose from the motor and from the wand, coil it up and store it separately nearby.
As to the bin, most Dysons have a flat bottom bin. When you empty the bin, the bottom drops away and the debris falls out. Well, with the DC39, the bottom of the bin is bowl-shaped, not flat. When the bottom is released, bin debris gets caught in the hinged bowl and doesn't fall into the trash can. You need to get in with your hand (or some tool - a pen will do) to hold the bin open and then angle the whole thing so the debris falls out of the bowl shaped bottom, too. It can be a somewhat unhygenic process, and you'll probably want to wash your hands after emptying the bin, just to be safe.
One other potential minor downside, the handle that you're supposed to carry the motor unit by is plastic that's about 1/8" thick and flexes noticeably when you lift the unit. I don't know if that means it's liable to break somewhere down the road, but it feels like it might.