I have an early 2011 MPB, with the stock 4GB of RAM. I just downloaded the new iSTAT Menu 4, which sits on my Menu Bar, and noticed that even with my relatively light use, I was using about 95% of Memory, and with just 1 day, 10 hours of uptime, I had 288,000 of page outs...so I figured some more RAM was needed.
I bought 16GB of RAM and a tool kit. My MBP is much happier now.
I'm considering going to 16 - though have 8 already so may not make as dramatic a difference, but I definitely notice some slowdown when using Photoshop with multiple layers.
How is it - the early 2011 model is still a really fast machine (I have the 2.2 model), so I'm sure more RAM would make it even better.