After doing some tests with dynamic on my Home lab I ran into a problem that suddenly the performance of the parent partition was well below normal. So after some troubleshooting I figured out that my VM's were using too much memory. Normally I would just stop one of the VM's and reduce the amount of memory to that VM. But after installing SP1 this fix didn't give me the desired results. I soon found out that the dynamic memory feature was the cause. There just wasn't enough memory available for all VM's I had running.
Normally Hyper-V will calculate a parent partition reserve but this calculation is done assuming you are not running anything else on your parent partition. Off course this is best practice in any production system but on my home lab I was running other software on my parent partition.
Fortunately Microsoft has a registry key (HKLM\software\microsoft\windows nt\currentversion\virtualization\memoryreserve, a DWORD) that will let you tweak the parent partition memory reserve. Now I can reserve more memory for my parent partition and keep the software running. Downside is that I have less memory available for my VM's. But off course dynamic memory isn't a magic tool that will give you more memory, it just will use the memory more efficiently.