I just used ramdisk on my minecraft, minecraft now runs twice as fast. With optifog/optimine - oh god I already nerded all over the keyboard. It's messy.
RAMDisk does improve loading world times and a little FPS, but it isn't really needed. I used to use a RAMDisk to run my saves off but whenever I start a new world, it'd be more laggy without the RAMDisk!
If you scroll down a bit on the first page you'll see it.
Basically its a mod that fixes the Lag Spikes of Death that have occured since Beta 1.6. All it is, is the game auto-saving itself every 2 or so seconds, which is causing Lag Spikes.
So in all, the Fix, Fixes Lag Spikes.
Sorry but this experience was around 1.3-1.5 I haven't tested recently because I can't enjoy minecraft as much without this mod. I cleared my whole world of trees by setting them on fire so I haven't tested it.
Leaf decay was fixed for 1.6, currently it is performing pretty well.