I have a 440; I've had it ever since TechShop went belly up and I couldn't use their 1100 anymore.
In general, when I take easy cuts in aluminum, it works just fine; I don't seem to have the Z or Y axis chatter that some people complain about, and the finishes are good enough for my needs! Full enclosure with flood coolant and power draw bar makes working with it convenient, too.
Today, I was slotting in 303 annealed, using a 3/8" (Maritool carbide) end mill in high quality ER20 collet, using a 1.8mm stepdown (that's 70 thou for you still stuck with imperial.) Using flood coolant to clear chips, which works great. However, on the second pass, it bogged down, and stopped, and I'm somewhat annoyed at this. I was spinning at 3000 rpm with the belt on the low-RPM range (so equivalent to 6000 rpm on the high range) and feeding at 13.78 inches per minute.
Formulas on the interwebs, and GWizard, says that this cut (this MRR in this material) should use about 0.36 horsepower, or 270 Watts. The spindle is rated at 500 Watts, or 2/3 horsepower, so this cut should have been only half of what it's supposed to be able to do. I thought I took it easy, but yet another end mill and yet another piece of stock goes in the trash. I have similar problems with fly cutting; cuts deeper than 10 thou will often just stall up (although the insert and even stock is often salvageable in this case.)
So, is there really that much difference between 3000/lo RPM compared to the optimal RPM in horsepower? What would the optimal RPM be? Or, (and this is my pet theory) is the PID controller for the VFD for the spindle too slow to react, so it doesn't have time to add more power between one alternation and the next of the BLDC motor, and thus once it stops up a little bit, it's in a death spiral and immediately seizes up? Basically, it has to be ready to dump all power into the windings on the very next commutation if it detects that the spindle is even a little slow; it doesn't feel like it's doing that. (Motor control nerding here :-)
I understand that a small mill that fits in my laundry area is not going to be a 10 kW monster; I'm more interested in understanding exactly what the limitations really are, and what the optimal choices are for RPM, and whether there perhaps exist patches for the VFD to make it react faster? Any experience/advice would be welcome.