Hi there,
I have a fair amount of experience with Apama... I worked for them for four years and still use the product in my current job.
I wouldn't say it's necessarily aimed at institutionals although license fees do tend to make it prohibitive for individuals. However, the client base ranges from tier one investment banks right down to small hedge funds. It's not just finance either... it's also used in transport, telco, retail etc.
At the core, there's the event processing engine (the Correlator) and a very powerful, purpose designed event-oriented language (MonitorScript). Around that you have a huge range of tools, utilities, APIs, frameworks, adapters etc. There's also an IDE with integrated debugger/profiler and high-level environment for building components/applications visually (Event Modeler).
So yes, it’s a large and fairly complex platform but it can be used to implement large scale enterprise systems (e.g. a trading exchange) as well as small scale applications (e.g. a trading strategy).
-m