Jeff's thoughts mirror my own.
I print via Image Print on my 4000, and I think a RIP is essential in order to get the best quality from this printer; even more so now that I have upgraded to Leopard and there is only limited driver support in Leopard for the 4000. ImagePrint supplants the epson drivers with proprietary (I'd say, much better) ones, bypassing the problem.
I have used IP's workflow features some in the past during high-volume work situations, but not so much any more. Most of my work is fine-art, one-at-a-time stuff so I can afford to go slowly! For me, therefore, IP serves as a pricy third-party printer-driver upgrade. That said, what it does, it does extremely well. Great B&W was my original goal in buying IP, and it delivers in spades.
I'm hoping to move to at least a 24-inch printer in 2008, probably the HPZ3100, unless Epson makes a comparable printer that allows simultaneous installation of both photo- and matte-black ink cartridges. The 4000 will get sold or used with a specialty B&W inkset. I do not foresee buying ImagePrint for a new printer, given the prohibitive cost and the excellence of the latest drivers for those machines.
I doubt I'd buy it if I were purchasing the 3800, from what I know of that printer.