The device pretty much takes whatever I throw at it. Any issues seem to be Android related, not really device related. I'm currently running the 2.0.1 firmware and I find that it's less stable that the 2.0 firmware. I occasionally have spontaneous reboots with the firmware that I didn't have before. The 2.1 milestone firmware is currently being deployed, hopefully that will fix up a few issues. I've installed task management applications and occasionally use them to kill running process, but that's more out of curiosity than anything else, I haven't done it because I've experienced any device slow-down.
One complaint would be the pace at which updates are released – 2.1 was released some time ago but it's only reaching us now. That being said, I think HTC is just as bad, possibly worse, I also have an HTC Dream and that was left on 1.1 for ages. I still think the Dream and Magic are officially running 1.5 and the Hero 1.6. While Motorola's update schedule is slow, it isn't that slow that I've felt the need to hack my phone – it took me two weeks before the Dream was rooted. The software as it ships on the Milestone has been fine for my needs.
Another complaint would be the touch buttons on the front. When the screen is in power saving mode, so are these buttons, which means you either need to open the phone or hit the power button to wake it. Same deal with the Nexus One except there you have a trackball you can hit.
On a plus side, I do find myself using the physical keyboard more than the soft one. Simply because it's easier to switch case and special characters more easily, it takes too many keystrokes on the soft one. Although I assume if you only have the soft keyboard available you must get quite quick with it.
The final factor for me was price – I could buy this phone for just over R5k in Jan, which was even cheaper than the HTC Hero at that stage. HTC phones seem to be a tad expensive here.
The HTC's have also become a bit hacker unfriendly, they started off fairly open but have since got worse with their “perfected” bootloaders. The last time I checked the only way to update the software was to exploit kernel bugs to gain root access and then re-write your partitions. I haven't had the need to hack the Milestone yet but it seems as though it may be more open looking at the updates for the Droid (US version).