There are many positive reviews of the at4021, check out Mike Jasper's article in Tape Op 2009. Most of the positive comments also apply to the at4022 (the omni version of the at4021). I record acoustic music only, and I need to record the natural sound, as clear and detailed as possible. For my purposes the at4022 is my go-to mic for an sd omni.
In addition to recording soloists or pairs, I sometimes record larger groups, and here again, the at4022 excels.
One reason I like this mic so much, is that I have had excellent results close micing instruments (avoiding proximity effect), but equally good results when positioned well back.