All the dealers say it's no big deal, but that's because they're in the business of selling tractors if you ask me. If you happen to run across a dealer who still has a Tier 3 on the lot, they'll give you the song and dance about how great that tractor is because it's the last of the Tier 3 before 4 or 4 interim hit.
In my mind, the DPF is the key here. Some can be baked out a few times to clean them, eventually though, they will need to be replaced. I talked with a Kubota dealer who said that a replacement DPF on an 80 - 90 hp machine was around $3,700 in parts alone. Real world experience is still to be determined on what effect it has on longevity or fuel economy. I know what the DPF did to the fuel economy of the pick up truck market when it hit in 2007. Adding urea has gotten it better, but that's still fairly new too.