Spinlocks, How Useful Are They?

It depends on what you're doing. In general application code, you'll want to avoid spinlocks.

In low-level stuff where you'll only hold the lock for a couple of instructions, and latency is important, a spinlock mat be a better solution than a lock. But those cases are rare, especially in the kind of applications where C# is typically used.

In C#, "Spin locks" have been, in my experience, almost always worse than taking a lock - it's a rare occurrence where spin locks will outperform a lock.

However, that's not always the case. .NET 4 is adding a System.Threading.SpinLock structure. This provides benefits in situations where a lock is held for a very short time, and being grabbed repeatedly. From the MSDN docs on Data Structures for Parallel Programming:

In scenarios where the wait for the lock is expected to be short, SpinLock offers better performance than other forms of locking.

Spin locks can outperform other locking mechanisms in cases where you're doing something like locking through a tree - if you're only having locks on each node for a very, very short period of time, they can out perform a traditional lock. I ran into this in a rendering engine with a multithreaded scene update, at one point - spin locks profiled out to outperform locking with Monitor.Enter.

For my realtime work, particularly with device drivers, I've used them a fair bit. It turns out that (when last I timed this) waiting for a sync object like a semaphore tied to a hardware interrupt chews up at least 20 microseconds, no matter how long it actually takes for the interrupt to occur. A single check of a memory-mapped hardware register, followed by a check to RDTSC (to allow for a time-out so you don't lock up the machine) is in the high nannosecond range (basicly down in the noise). For hardware-level handshaking that shouldn't take much time at all, it is really tough to beat a spinlock.