I bought a 1984 Ericson 26 last August and the first thing I did was rip out the head and the smelly, awful bladder holding tank. I replaced it with a Thetford 550P MSD porta potty, which I plumbed into the existing system. I'm really happy with this setup. I've had no problems with odor and the capacity is more than I had with my Catalina 30. Although the tank on the Thetford is 5.5 gals. vs. 14 gals. on my Catalina 30, the Thetford actually winds up holding more because it uses far less flush water compared to a conventional head, in which you have to pump a slug of water into the system just to move the waste down the hose into the tank. The porta potty, by contrast, is little more than a glorified bucket, and a quick rinse is all you need to do the job. So, for example, on my single-handed trips to Catalina I could go roughly 5 to 6 days before needing to empty the tank back when I had my C30, but with the porta potty I can easily go one week.
Being plumbed into the system I can simply dump it overboard when I'm 3 miles offshore. Right now I have it plumbed with a Whale manual waste pump but I hope to convert it to a macerator, making overboard discharge a push button affair.
Another nice feature is that there is no doubt about how full the tank is on the porta potty whereas on a conventional holding tank you may not always know, unless you hook up some kind of gauge (which I had on my Catalina 30). Even with the gauge, it's not as accurate as the "visual inspection" of the porta potty--even though the latter may be a tad less appetizing!

Oh, and the porta potty does actually have a gauge on the front as well.
The only complaint I had was that the mounting hardware that came with the Thetford was totally inadequate. My friend Neil (who sometimes posts here as "Old Skool Neil") came up with a nice design for latching it down and he had a friend of his make it out of stainless. But that was custom for the dimensions of my boat. Still, I would think one could easily fabricate something similar in shape to the plastic pieces they provide and, made out of stainless or aluminum, it would be much more robust than the plastic hold downs that come with the unit.
--Alan Gomes