I've machined some holes in the brackets for the bed (see my mk3 build log) but don't want to do much more until the limit switches, e-stops, extra X axis limit switch, etc is made and set up. It's too risky to run in this condition.

From that limited experience it feels like the machine is going to be reasonably OK for aluminium in terms of build stiffness, but as per my other post on 'machining aluminium with a 1.5kW spindle' I'm having problems relating to having to use too high an rpm and melting the aluminium to the tool. I was running at 0.5mm DOC, 300-600mm/min feed, and 10,000 - 12,000 rpm (!). The surface finish on the inside of the holes looked fairly good but I can't comment further without cutting doing something a bit more demanding.

I won't be machining anything else for a bit whilst I finish bits off so hopefully someone else can make similar measurements and comment further if you need a better answer sooner.