I am reading this thread and am fascinated by the fact that most of the posts are from people who have never owned an IPS powered vessel. As an owner of a 40pc that is currently being repaired for the infamous "water intrusion" let me give you some thoughts. First, this was my first diesel powered boat and I would never go back to a gasoline powered boat again. The fuel economy, trouble free operation, safety, cruising speed and longevity of a diesel engine cannot be compared to gasoline powered vessels especially when you get above 30 feet. As far as IPS is concerned, this was the first problem I had with IPS drives since I purchased the boat. After speaking with other IPS owners I have discovered a little known fact, lol. While IPS owners, who do have problems with their systems, complain, the majority of owners with problems (other than water intrusion) has either run aground or haven"t serviced them according to Volvo recommendations. Yes, service on IPS systems are more expensive but that is because they are bigger and more complicated than simpler outdrives or V-drives. Speak to any large boat owner and ask about the cost of repairs on large diesels or surface drives when an object is struck. By the way, other than water intrusion, almost all IPS installations are trouble free.
Now back to water intrusion in IPS drives. When Volvo Penta first released them, it seems they made a mistake. The steering seal was made of bronze and after a period of time, it corrodes and the seal begins to leak. Volvo has since redesigned the seals, both steering and propeller shaft seals. They are now manufactured from 316L stainless. When drives are serviced correctly, the bronze seals lasted about five (5) years, the stainless steel seals are expected to last two to three times longer. If the repair falls "out of warranty" it is expensive (approximately $11,000). Because of this, some IPS owners don't repair their vessels when water intrusion is discovered. They simply replace the drive oil and keep running their boat. That is what causes catastrophic failure.
Lastly, the directional control, ease of operation, fuel economy, cruising speed and quality of support from Volvo Penta cannot be matched and I would not go back to "normal" propulsion after owning an IPS powered boat.