I’ve got the 420’s on my track bike and the Sprint350’s on my road bike. The 350’s are ace; very light (within a few grams of the much more expensive Ksyrium SL’s) and amazingly stiff. Not sure how aero they are but the V-section rim is good for stiffness while still allowing use of normal tubes rather than specific long-valve ones.
Only thing to watch is the bearings. The shop had to strip and rebuild the front hub from new cos the bearings wouldn’t tighten up properly and the wheel was rattly in the drop outs. I had to retighten them again about 3 weeks later. Since then they’ve been trouble free.