What exactly is it you want silverlight to do for you ?
Silverlight is Microsofts attempt at Adobe flash and is not as widely supported accross platforms so you may be better off just going the adobe route.
There is no harm in playing with it to see what you think, the best environment for developing in is using Microsoft Expression Blend.
You can download an evaluation of it from here:-
Microsoft® Expression®
Of course if you are just on about your web experience then by all means install it as sites that have the functionality will be slightly more interactive than not having the plugin installed.