Right now I am debuggin a multiserver environment with Debian Squeeze (2 NSs, 1 Mail server, 1 Web/DB server). As for ISPConfig3, I didn't found any problem yet, but I think I will be testing it for the next one or two months...
I have a problem right know with DNS resolv (some hosts resolvs, other doesn't, it depends of the ISP... if I use google DNS I haven't any problem at all, it works perfect... maybe it is a update/cache problem of the Argentina ISP's DNSs... we will see...).
I think that I will not found major problems with the ISP3 setup... right now, I recommend to install and test it by yourself.
I am not using server clusters at this moment, by I plan to use in one or two years... I have soft raid working in 4 servers (RAID1) without problems.
Be carefull with monitor of ISPConfig, it reports all your servers errors with services, but... if you shutdown a server, it will be reported as "ok"... (this will be fixed in the nexts updates os ISPConfig I think).
Please, if you choose this way, give me feedback of what you found!