Who exactly is this "bunch of people"? What are they comparing it against?
For the vast majority of things, C++ is not "much faster" than C#. It certainly has benefits in various situations, particularly where you want more deterministic memory handling, but in my experience the bottleneck in most applications isn't in places where C++ would help. As spoulson says, a lot of performance is in the design instead of the exact implementation - and there, it helps to be able to try different designs easily.
Why would we use C# when it's a bit slower than C++? Because it's generally reckoned (i.e. some disagree :) to be a lot easier to develop in without shooting yourself in the foot.
As for what C# can be used for... what do you want to use it for? Unless you want to develop drivers and kernels, it may well be fine for you. (Even OS development has some folks using C#...)
Job opportunities? Loads.
Downsides? Well, .NET itself is only available on Microsoft platforms. There's Mono, but it doesn't have quite the same degree of portability as Java (no doubt another "slow" language according to the same bunch of people).
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