When I replaced my 24V spindle with the 611, I ended up having to shim the top of the Z axis out about .097" to account for a ~.012" ‘step’ when I was using my 3/4" wasteboard cutter. Without shimming, the offset would have been barely noticeable using a 1/8" bit, but the 6x larger bit made it more visible. All I had to do was replace the screws with some screws that were a little longer and use pieces of business card as shims. It was very easy to do.
As for the 24VDC spindle, they had a batch of bad spindles that accidentally got sent out to customers. After 8-12 hours of use, they would typically start to fail, causing lost steps from the electrical noise and drastically increasing the runout. They were able to get a corrected batch, but I think they decided that it would be better for the customers to get something a little better. The spindles were very low on torque and speed, and made jobs take a lot longer than necessary. I actually compared the 24VDC spindle to the 611 in this thread: 24V Spindle vs. 611 Router - Quantified. The 611 project was 7 to 17 times faster, and the cuts on the bottom and edges were all cleaner with the 611 than the good spindle.
If you’re milling aluminum, I would suggest using the 611 or a similar router. I doubt the 24VDC spindle could handle it, but the 611 did an aluminum job just fine for me. I used a .125" end mill and was able to cut it dry with .1 mm deep / 800 mm/min passes. The edges were beautiful.
Runout on the 24VDC spindle would increase if you pushed it too hard, because it’s pretty weak. I don’t know for sure what the runout on the 611 is, but you can always try to minimize it by using the precision collets from Precise Bits at http://www.precisebits.com/products/equipment/dewalt_611_kits.asp?tsPT=!!!Dewalt_611!!!Kit!!!PG!!!. There are other collets out there from companies like Elaire, but Precise Bits had the highest tolerance.
The X-Carve will do some precision work once you get it dialed in, but remember that it is primarily a hobby machine. If you are looking for the precision of a $10,000 machine, you’re not going to get it. But the X-Carve will get you darn close, I have mine dialed in to cut within .005", often a little closer, and am always looking for a way to get it a little better.