I have been using ImagePrint (v6.1) on my old G5 machine for years, with my old Epson 7800 set up as a Phatte Black (dual black for matte and glossy surfaces without changing inks), and get uniformly outstanding and repeatable results on almost any paper or cloth I choose for printing. The ink use is actually less with a proper paper profile than on my Epson 4000 that does not use the RIP, and has a very similar ink configuration.
A good RIP provides several features that have not generally been available for most printer/app combinations. For starters, the ImagePrint RIP actually reads files in full 16-bit output. Some of the newer printers and Mac OS versions are now handling things this way also, but RIPs have done it this way from the start. The possible layouts and printing controls in a RIP are far more flexible than any of the drivers provided for printers, or even in the various apps such as Photoshop. The ability to print on almost any type of paper or surface with well-tuned profiles was also not really available without either doing a lot of profiling on your own for a limited number of papers, or now using something like the ColorMunki, that still seems to have some challenges.....especially if one is at all impatient ;-) I routinely switch from B/W matte to color luster or glossy, or even canvas (glossy or luster, fine or coarse), or films or textiles without any loss of color fidelity and rendering, other than what the material permits. (A warm based matte like Epson Ultrasmooth will never give you the whites of Professional Luster or some of the other papers with or without optical brighteners, but the rest of the colors will be very true across the board, from my experinece.)
Jack's pointers about how to set things up in the print controls without using a RIP are critically important. If you do not get the color management set up correctly, you will have lots of issues. On the other hand, with a RIP, you do not have to worry about any of that, as it is all properly managed in the app. I create 16-bit PSD files as finals, and then drop them into a folder for printing. From there, the rest is a near perfect, and always repeatable process, regardless of paper type being fed in.
RIPs are not cheap, but they do a fantastic job, especially if your set-up changes. I routinely make new prints or make reprints for folks at differing sizes and times and every single one matches every other one. From my commercial perspective, that is terribly important, and I think that would be the case for anybody doing repeated printings over time.
All that being said, I do think that some of the newer printers and associated gear, plus some of the better profiles being created for these printers by the OEM or with their onboard tools, one could easily do without a RIP if your printing needs do not require lots of different surfaces and sizes. I may forgo the RIP with my next round of computer/printer/app upgrades, but I am already preparing for the pared down choices there.
LJ