Summary: This lens is a compromise, which is not a bad thing. Especially since it is a very, very, good compromise. It is light enough to carry and hike with, small enough to go in your carry-on on a regular airliner, long enough for many (not all) wildlife photography tasks, and fast enough to function well in low light, or to keep functioning reasonably well with teleconverters. I find it an especially good "big gun" to take when traveling by air.
Pros: light weight and compact for a super telephoto. Superior sharpness.
Cons: price, you do need the teleconverters for wildlife photography, not long enough for birds (except big tame waterbirds).