I think all the consumer-grade backup and sync services are designed for a relatively small number of large files, such as high-res photos, songs and videos.

I have about 300K files (250GB total) in my 1TB SpiderOak account. The UI has always been atrociously slow, but backup speeds are okay when most files are static. Things get more interesting when I check out another branch in git and touch a few thousand files at a time. As for sync, well, I've all but given up on editing a file on my desktop, opening my laptop, and expecting the updated file to appear anytime soon. If I really need to sync a file immediately, I put in in my Dropbox (free account, almost empty).