The Tanzania coffee character belongs to the Central/East African family of washed (wet-processed) coffees - bright (acidic), and almost aggressively flavorful. Kenya may still produce superior coffee, but Tanzania has pushed ahead and shows many of the same positive qualities of Kenya. Coffee is grown in the North and South and there is good quality potential from both areas.
Peaberries are often sorted out and sold at premiums, but the cup is sometimes tainted and not worth the price. It has become a novelty coffee, and sells well in the US, so many roasters capitulate. Yes, it is a coffee with great potential but shipments arriving in the US do not always express that truly excellent Tanzanian cup.
One possibility is the coffee ages (or is steamed essentially) in stalled shipping containers on its way to port. Every so often I cup a really good example of this coffee, but some years are a complete bust. The problem is that Tanzania has realized it gets a premium for its peaberry no matter the quality ... so what's the incentive to actually pick and care for the coffee better, in order to prevent this defective character?
Blackburn Estate from Ngorogoro has been a consistently good arrival, the highest rated Tanzania in recent memory. Ruvuma district has also produced a solid cup. "Northern" coffees are considered to be more "generic", and Kibo has that off, baggy, "steamed in the container" note. The Southern type is a clean cup, zesty, albeit mild next to the Kenyas. In the past, we had micro-lots from Nkoanekoli and Ngorongoro that represent progress from the other regions.
I guess what I'm saying is, there is a definite range of quality coming out of Tanzania, with many top lots suffering damage during transport. So keep in mind that if there is a current Tanzanian offering listed here, not only did it have to overcome many environmental inconsistencies, it also had to overcome my cynicism and must be pretty damn good.