I say slow. Speed can and will hide a multitude of technical defects, while practicing slowly means everything is on show.
Here's an excellent explanation from Ryron and Rener Gracie's site:
When practicing, always execute the techniques so slowly that it is impossible to make a mistake. The slow pace and predictability of proper training will provide your partner with many opportunities to counter the technique. Again, your training partner’s role is to consistently simulate the most common behavior so that you can perfect the techniques – not to fight with you. Eventually, your diligent and exacting practice will produce precise, efficient, and quick reflexes that will leave your attacker with no opportunity to counter your techniques. In a real fight, you will also have the advantage of surprise since your attacker will have no advanced knowledge of how you react to his actions.
As you gain experience the pace at which you can be correct, precise and efficient will gradually increase. This speed-up should happen naturally. By the time gradings come around it's not unusual for the panel of black-belts to have to ask candidates to slow down so that can observe their technique -- they may have started slow, but by grading time they are a blur.
By practicing slowly and cooperatively, we can practice dangerous techniques safely, develop sensitivity, fluidity and precision, and not kid ourselves in the process.
That said, I do train for speed as well. When practicing hung kuen kung fu sets, besides practicing at normal speed and really slowly -- think taijiquan (tai chi chuan) speeds -- I also do some sets as fast as I can. Similarly for punching drills. But these are individual drills (safety first). In jiu-jitsu and judo, when I'm thrown I get back up and re-engage as quickly as I can.
Another oft-cited reason to train fast is for the workout, but slow methods can be equally (or more) demanding. For example: Try doing a set or drill very slowly using only low-to-the-ground stances.
What's your approach to slow vs fast (and justification)?