Besides being much slower than the vsphere client, there are three other issues to be aware of with the web flash client:
1). Unsoliciated updates to the status of VMs and information stops after a few minutes, with the exception of if you have a monotoring screen up (e.g. CPU, disk ,etc.). If you start or stop a VM it will apparently poll the VM for a few minutes and accurately reflect the fact that the VM evenually started or stopped; however, if you have the running VMs up and showing, and you shutdown the VM through some other method other than initiating it from the flash client, it will not show the change of status until you manually refresh the screen. Therefore, you can't just leave it sitting say on your second monitor and expect it to have up to date information like the vsphere client does. This is a significant loss of functionality versus the vsphere client.
2). You will get logged off for inactivity and there is no way to turn that off. The vsphere client stays connected indefinately.
3). The flash client appears to eat up a lot of memory within vcenter server and it looks like there may be memory leaks in vcenter server as I see the swap file size usage keep going up the longer you use the flash client (especially the monitoring features), and this space isn't reclaimed if the client logs off. This has gotten somewhat better under the current release that came out in December, but it's still present.
