Handled one only once - but didn't get to shoot it.
My reservation about the HK21 is the calibre since most countries are trying to go one calibre and the 7.62mm x 51 is a holdover from the '50s. Don't get me wrong, good bullet. Me like too.
With the idea of weapons systems (reduced training, simplified logistics and parts), the HK doesn't fit in unless you're using HK rifles too. Then again, like David and you said, it could be a price thing.