I used the 15x50 IS for several years as my primary birding binocular. There was a lot of medium-long distance raptor migration watching involved in that. It worked very well, and regularly allowed me to spot and identify raptors at distances fellow birders using 8x-10x binoculars would need their scopes to either find or identify them. A good scope, of course, would have a much greater reach still.
More critical focus is an issue when going quickly from near to far, but can be overcome with some tricks.
1) I put an "O-ring" between the focus knob and the binocular body to prevent focus creep when the binoculars are hanging down. That was because the focus action is light enough that the objective lens system, which moves when focusing, would creep slightly outwards, towards near focus, while the binoculars hang on your chest. The O-ring is easy to pull off if and when you want lighter focus for forest birding.
2) Index the focus wheel with a marking that shows when there is infinity focus for your eyes.
3) Focus on some terrestrial target at the distance you expect the flying raptors to be at before you begin scanning the skies.
4) Use Eneloop X rechargeable batteries, and charge them often. They will work for a lot of hours of consecutive IS-assisted birding, but it is nice to know they are not going to run out on you and the IS works a bit better when the power source has good current capacity.
5) Get a selfie stick and attach it to the convenient tripod thread under the binocular. This takes a lot of the strain off your arms and shoulders, and makes long periods of scanning the skies easy and comfortable.
I eventually sold the 15x50 and replaced it with the 10x42 L IS, which has better image quality and a more convenient all-round magnification/FOV/DOF combination, but the 15x50 definitely showed significantly more in distant raptors - or any distant birds for that matter.
- Kimmo