I've had an Infuse before and that 4.5" phone has almost the same dimensions as this and it was quite manageable. The biggest problem was that the cheap plastic did not inspire confidence when held, and the biggest were the constant worries of bending or squishing the phone even in a large cargo pants pocket. I can go thin and big, just not thin and big AND cheap build. The Titan seems to solve the build quality part at least with its rigid metal chassis.
The HTC Titan is currently on my shortlist with the Nokia Sea Ray. I want to try WP7, and DON'T want anything made by Samsung (had them for like 5 phones already...).
The Titan though is far from ideal. For one, the design is one of their blandest to date IMHO. It looks like a prototype to test out software and the hardware (aesthetically at least) was just slapped together in five minutes as a placeholder. I was hoping for a smaller phone with a Mozart design, with the rough edges that phone may have had ironed out and maybe a 4", but instead we got the Radar/Trophy 1.01...which I find really ugly and has a bad screen. Hoping for more WP7 devices to come from HTC besides these though. The other problem the Titan might have is the camera. HTC is pretty much at the bottom of the barrel when it comes to video capture. The MyTouch 4G slide had a similar specced lens as the ones being touted right now and the video quality (and audio capture) was pretty bad. Hopefully this is a different setup and they have made leaps and bounds on this front in the last couple months. Hopefully the speakers are up to snuff as well on a <10mm phone...