They're the perfect size I think, hundreds are too big.  It is a decent amount of money yet you don't need to worry about smaller retailers not having enough change.
I also like them partly because they're less common.  :laugh:   I prefer $10's over $5's and $20's for the same reason!
And lastly I just like the way they look.  Grant on the front, the Capitol on the back, it's a nice looking bill.  My favorite bill ever was the old $10 bill and I have a few of those which I don't plan on spending unless I face a dire financial disaster.