I haven't replaced mine yet (see my earlier post - I bought the Crucial M4 256GB) but I can guarantee you that you will notice a HUGE improvement in performance and speed.
Boot/shut down times will be much faster. HD's need to spin up before they can be accessed. Data can only be read/written when the the disc is in the right position under the HD pin/needle. SSD's can be accessed MUCH faster. My stock HD's access time is 12ms. The Crucial M4 256GB is <0.1ms! That's just the speed!
They have no moving parts they are absolutely silent and much more shock resistant/durable. They use less power and generate less heat. I can't think of a reason why you shouldn't get an SSD apart from the cost! If you can afford one then BUY ONE!! Once you pay the price then it's all good from then on.
I just ordered mine from Amazon and it's supposed to come by Tuesday (the day before ML is supposedly coming out so perfect, I'll install it when I get ML together). I'm excited now. Thanks!
It was pretty expensive ($203), but I rarely spend my money on personal things like this, so I decided to go for it and hopefully it'll pay off!