Hi,
Just changed my phone from my (almost) two year old Desire to a Defy Plus (more or less the same phone as original Defy but with a faster clocked processor and a slightly bigger battery).
I use my phone as bike computer on the bars (have done so for ten years) but getting waterproof cases is a phaff so wanted a waterproof one and Defy Plus fitted the bill.
I bought it from Buymobilephones.net and unfortunately the first two they sent were faulty (touch screen and random rebooting issues) but were replaced without any quibble.
The third one seems fine so far (4 days use). Initial impressions are it is better than the Desire in terms of its slightly smaller size and the screen is a bit sharper and easier to read in sunlight. It is a bit lighter and the ruggedness is ideal for outdoor uses.
For normal use it does everything as well as the Desire did. I can't really see any difference in performance (thought my Desire was running a custom ROM and overclocked and I am still running Stock on the Defy, though rooted) of day to day use. Such is Android's ability to be customised I am able to get pretty similar look and feel to the Sense interface from other apps. The only thing I am missing is a decent email widget to view multiple accounts (Launcher Pro's Gmail widget covers one account though).
Phone call quality is okay, the Desire is maybe slightly clearer but not much. The loudspeaker on the Defy is much better, with Desire I had to use a plug in speaker to watch News 24 streams etc if I was not holding the phone. The Desire camera is a bit better due to its focusing but I don't use the camera that much so it is no big deal.
As the Defy is waterproof I no longer need a waterproof case for the bars. I have simply modded an aluminium case (Pdair one) by drilling a couple of holes in the bottom of it and cable tieing it to the stem. Overall it is a good bit smaller than the waterproof case I was using for the Desire. It is a bit bigger than say a Garmin 800 but not by much. Used it yesterday in 3 degrees cold with a small shower and it survived fine.
Battery life on the bike seems quite good. Using RunGPS with the screen on auto brightness and set to go to lowest brightness level after two minutes I could only get about 3 hours out of the Desire. Based on an hour and half run yesterday the Defy should easily get to 6 hours which is more than enough for me. By setting the screen to turn off completely you could probably get about 8 to 9 hours.
So as long as it holds together it is a good alternative to the Desire with the added bonus of being waterproof.
Regards