When it came time to upgrade from our Seafarer, we wanted 4-strokes on the next boat (we kept the old one 10 years and wanted to do that again). The best boat we could find had HPDIs and I did a ton of research about them. The general concensus was that with rare exception they were some of the best engines Yamaha has ever made.
We've had zero issues with two of them, though only for 2 years so far.
The good: Always start and light-up quickly. No smoke. Great power/torque. Lighter than 4-strokes. Burn VERY little oil. Equivalent fuel efficiency to 4-strokes (we routinely get over 2 mpg with a pair of them).
The bad: While quieter than our old OX-66, they're still considerably louder than 4-strokes. While smoother than any 2-strokes we've ever owned, they're still rougher than 4-strokes. When it comes to resale, most people are looking for 4-strokes only (we don't care, we keep boats too long to matter).
Overall opinion: Excellent motors with great power and efficiency. Can be a relative "bargain" compared to 4-strokes if you're willing to live with a little more noise.
