Have no real experience of Ensim but I have heard that it is hard to upgrade PHP and such things and is reliant on the vendor. Sounds strange and not sure if that's true but if it is, would be a huge drawback.
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I have experience with Ensim, it breaks easily. Slow. And sometime when you create a new domain name in Ensim it freeze or httpd has to be restarted. You cannot use the email hosting account straight away after creating it, somehow it will be ok the next day or after sometime. If you want immediately, restart sendmail, appliance or the server.
You need a server with lots of ram if you want to put many client on Ensim. Those who like a secure CP Ensim is the one, but it pain when you want to work fast on Ensim, it lock you out from one screen to another. So enjoy the pain if you are using Ensim.
Do give Plesk 7 a look. I have enjoy it since I installed it on a P4. GUI is fast, run smoothly, no tweaking of config file so far. If I have a chance I will update my experience with plesk after I have all my clients move to plesk server.