I have been using imageprint for about 5 years. It has more than paid for itself. Layout options, print stage sharpening and corrections, text (for example - to include a hi res image title at the bottom of the print). These are all reasons I use it. Their support team (Daniel and Irene) are amazing as well.
Recently, I have been curious as to whether I can have better color profiles made. I had some made by the well regarded Chromix company. While the prints are reasonably close, they are different. I have only done a few test prints, such as fall foliage. So far it appears that I have less out of gamut color with imageprint, and (perhaps as a result?), a little more detail in certain areas. This is confirmed by comparing the image print profiles to the other profiles for various papers - using Photoshop proof colors with gamut warning turned on. I also compare the prints themselves.
You can make your own color icc profiles and use them within imageprint. But, even if I were able to develop better profiles, I would still use ImagePrint for all the reasons I mentioned above. With regards to cost, the no-charge profiles makes a huge difference. High quality profiles from profiling services can cost you up to $100 each. While I have 4-5 papers I generally use, I often use various other papers for specific projects. That would add up for traditional profile services, and then when you change your printer, you would have to start all over again. That is not for me 😀
Regarding Canon, the do support the more recent models, but I believe the direct hardware support (bypassing the printer driver like they do with Epson) is coming in the next few months.
BTW, I have a P800. If only image print could fix the need for Epson black ink swap 😀