If i've got to have thottling/caps, then Virgin's solution is the best i've seen so far. I get the speeds I pay for (10Mb/s) 24/7, It's easy to schedule big downloads overnight, and even if you do need to download something big in the daytime being capped to 25% normal speed isn't that much of a burden. 2.5Mb is plenty for browsing and streaming flash videos, i wouldn't even be certain I could tell the difference.
If you're on 50Mb even when caps come in, they'll be big enough that nothing will hit them short of a 1080p movie download or a full game on Steam, and being limited to 12.5Mb after the majority of it has come down isn't going to hurt at all.