I've used Reporting Services and Crystal fairly extensively, and I'm writing a few reports using Excel(ick) at the moment.
Reporting Services is pretty good for simple reports but as soon as you need total control over formatting,complex formulas and charts etc. Crystal is a long way ahead. I also find Crystal to be far more usable; being able to change things within the report preview is invaluable (it may be possible in later versions of RS?).
RS also needs to be deployed to a web server which limits it's usefulness if you are writing applications that need to be deployed externally.
Older versions of Crystal were very buggy but the latest ones are much better, it's much more mature than Reporting Services.
