I just purchased a 2008 TCH with 28K miles. Changing the transmission fluid is cheap insurance and tells you alot about the condition of the unit. My fluid came out very dark and some kind of yellow substance was in the bottom of the catch pan. I would highly recommend that DIYs change your fluid at reasonable intervals. IMO a resonable interval is 30K miles or 2 years.
The cost for Toyota ATFWS was $7.78/Qt at a local dealer and it takes 4- Qts.

 

My fluid, after 91k miles was pretty much the same as new, no strange smells, good red color etc etc. I didn't take sample for testing, but I did notice nice pressure difference inside the casing when opening , it took good couple seconds of hissing and I think the negative pressure was inside the casing. I thought the transaxle had some venting, I wonder if it was just my car with broken vent, or there is no venting on TCH transaxle?
As a reminder: TCH manual says to change both coolants (engine and converter, same type fluid) at 100k, iridium sparks at 120k. I didn't see any mention on break fluid and tranny oil. The good thing is TCH doesn't have normal transmission, just whole bunch of planetary gears and electric motors , so the oil should run cooler and therefore last longer.
Not sure why dg370 had such bad oil in his transaxle only after 28k, but I think changing it every 30k is a bit excessive. Except for cooling electric motors, the oil in transaxle does the same job as oil in differential in any other car and those usually last forever.

 

Pete,

Mine came out looking nice and red but the problem with the Toyota Power Split Device (transmission) is the fact that the planetary gears, MG1 and MG2 all run in the same oil bath and that it is splash lubricated. Any small metal particles that are suspended in the fluid find their way into the bearings and windings of the motors where they can cause excessive wear.

The more technical guys on the other Prius forums (not this site) have sent a LOT of samples off at various mileages and have them checked for wear particles and lubricity, viscosity and the remaining additive package of the fluid. The general consensus is that a 50,000 to 60,000 mile change interval is prudent considering the failures of some transaxles whose fluid was not changed. I find it nice and simple to change, only costs a few dollars and is cheap insurance.

Your other replacements (both coolants and spark plugs) will be changed at the 100,000 mile mark on my cars as well as other items that I do just "because" Those would include the water pump (if not done before then) the serpentine belt, upper and lower radiator hoses, etc. I change these items out because they have a finite life, can cause you to become stranded when they go out during a trip and are what I call preventive maintenance. I hate being stuck on the side of the road and NEVER want my wife or two daughters in that predicament.

 

I'm following Prius chats with interest as well and I think Bob Wilson did extensive testings of transaxle oil. If I remember correctly, the consensus was that original Prius absolutely needs tranny oil change on regular basis, however newer generation using WS oil wasn't as bad and lasted much longer. It also depends on particular car and since mine looked pretty good, I'm not going to worry about it too much. I don't change any hoses, belts etc, unless they seem they need to be changed during visual inspections. For some strange reason, in my experience with many different cars original, factory installed parts last forever, as soon as I change them, even with same dealer purchased part, often they won't last half the time and that goes for hoses, radiators, master cylinders etc.

 

INFINITY..! For wont of a "number" symbol.