I currently use 6 pcs sync'd together, master clocked with a Motu Digital Timepiece synchronizer so that any combination of the computers can be brought online at any time in any order. Each Pc is WinXP, tweaked for audio. A couple of the machines are 2.5 ram, the others 3gb each.
Programs running at any one time range from Nuendo to Cubase,
V-Stack, Gigastudio, and a range of others. There are also three or four analog multitracks in the equation synchronized to the whole setup, but those aren't often used any more.
Signal routing ranges from piping audio or effects back and forth via lightpipe, or actual analog,
fxteleport, optical via vstsystem link (used the least)....usually to increase the number of software virtual instruments or effects that can run with virtually no cpu hit on any one pc.
At mix time, I often use several of the pcs simply for software reverb/effects, or as ports where the hardware reverbs/delays etc are physically hooked into the computers as effects returns (appearing in Nuendo etc fx returns). For example, I may have one pc running only BFD on Vstack, with 21 tracks of drums submixed there and routed back over to Nuendo on another machine. Another pc may only be playing virtual instruments triggered from Cubase, routed into Nuendo. No end to the routing options.
Sound cards are various Echo Laylas and Audiofires. Latency with low buffers is average 2 or 3ms with the Laylas, varies with fxteleport, and is 3-5ms whenever using systemlink with up to 5 or 6 of the pcs at once (again, I don't choose the Steinberg systemlink routing very often).
The system is rock solid and has been for a number of years. I attribute this mostly to the use of the MOTU synchronizer as master clock and the incredibly great drivers of the Echo cards.
As far as ease of use, after one sets up a synchronized system in the way that works best for one's workflow, it's just like one big single system. Power everything up and it just works.
It's not a simple thing to lay out "this is how to set up a multiple system". The parameters totally depend on what sound cards you have, if you do or don't have a synchronizer, if you do or don't have a network router etc (in the case of fxteleport), and of course how many computers you have.
Additionally, your pc host settings affect performance, the amount of "sludge" stuff you have on your computer(s) affect performance, your level of expertise on computer tweaking affect your performance, etc etc.
