I, too, dislike due dates for exactly the reason of the "slow but steadies". I don't have due dates, and it avoids a lot of problems, but definitely creates others. I give the students credit for the class based on whether they earn a percentage of the total number of points available. The points come from projects (a total of 10 points each), classroom behavior (including time on-task) (5 points available per week), and possibly other sources such as exams, safety procedures, 5minute warm-up activities, etc. The total number of projects that I will factor in will depend on how many projects were completed by the majority of the students. So, for example, if 11 projects were introduced in all, but only 5 students out of 35 got to the last 2, I would probably say that the total number of points for projects was 90. Then, the students who did those other 2 projects just end up with extra points towards their overall total.
The good thing about this system is that it allows students to work at their own pace, and gives them extra time if they truly need it. The ones who are behind because they goofed around too much tend to loose points in the classwork area, and often don't earn maximum points on their projects because of effort issues, while the ones who are just meticulous generally earn maximum classwork points, as well as maximum points on the projects they do complete, so they come out ok.
The definite bad part about this system is that multiple projects are always going on at the same time, and I am often pulled in 6 different directions! This was easier to deal with when I had and aide, but I no longer have that luxury due to budget cuts. However, I usually seem to deal with the confusion, sometimes even having the "speed demons" help the ones who are needing help on a project they already did.
Sorry this is so long - hope it helps!
-Lydia, excited because I just found out one of my students got honorable mention in the Dick Blick lino contest!!