I've been using Remember the Milk as a personal task manager for 5 years now, but I ended up being quite bored by the bad usability of its web interface (the Android app is fine). For that reason, I decided to do some research about the alternatives and found very good opinions on Producteev. In fact, after trying it for a few hours, I had to agree that Producteev's web interface is more beautiful and practical than Remember the Milk's.
However, Producteev is far from being as flexible as Remember the Milk. Actually, almost nothing is configurable in Producteev. One annoying example of this is the fact that any tasks defined as "all day" (no specific time) will have a reminder set for 5 p.m. by default. Since most of my tasks are "all day" (actually, "anytime" would be a better definition for them, but there's no such option), I would have to manually unset reminders all the time, if using Producteev. Not being able to do it while creating the task (changing reminders must be done after the task was added) makes the problem worse.
Another problem with Producteev is that you can't set a task description. This is also not possible in RTM, but you can use notes as a good replacement for that. Although you can also create notes in Producteev, they are not editable (!), so they can't really be used as descriptions.
Finally, there's something with Producteev's Android app that may look correct for some people, but that for me is enough reason to give up on Producteev: the notifications go away after you see them. This simple scenario would be enough to make me forget the milk:
- I set a reminder for 4 p.m., concerning a quick task X that I must definitely start between 4 p.m. and 6 p.m.
- I see the notification at 4 p.m., but I'm still busy with some other task Y, so I can't do X at that moment.
- I finish Y at 4:30 p.m. and don't remember X anymore.
- Even if I look at my phone several times between 4:30 p.m. and 6 p.m., I won't be reminded of X, unless I look at my full to-do list at some moment.