Every week after I give my lecture (and lab) presentations, I post my PowerPoint slides (unedited) on my course website as PDF files. I do this primarily so that students don't have to worry about copying down all the figures and complex definitions I include in my lectures, and it also lets students correct any errors they've made while taking notes. Posting the slides also provides the students with a handy review tool, and allows me to more freely introduce content that isn't in the textbook, since I know students will at least have my slides as a reference.
I've gotten a lot of positive feedback in response to posting my slides, and have even seen students print out the slide sets, cut out important figures or definitions from them, and tape them directly into their notes. One student even used to leave blank spaces the size of my printed slides in her notebook while taking notes.
At the beginning of every semester, however, I get requests from a handful of students who want me to post the slides ahead of my lectures, instead of after them. This isn't completely unreasonable, as I know a number of my peers do this, and some even sell bound copies of their lecture slides in the bookstore (how those instructors manage to get a full semester's worth of lecture notes finalized before the semester begins is beyond me). While I can understand that this could work for some instructors, I don't think it fits with my lecture style, and thus, during the first few weeks of classes, I get to repeatedly explain why I don't post my slides ahead of time. Here are my reasons:
1) Posting slides ahead of time kills the flow and surprise of a lecture. I try to make my lessons as engaging as possible, and often include neat little surprises or tidbits of information that (I think) spice things up. If the students had all that information on the slides in front of them, the lecture wouldn't be nearly as engaging, and it would also encourage an "all right, just get to the next slide already" mindset.
2) I regularly edit my slides up to a few minutes before I give my lectures, and thus anything I gave my students would necessarily be different from what I actually presented. This could easily cause confusion about what material the students actually had to know, as well as making the lecture more confusing ("But we don't have that slide in our notes!").
3) I like to be flexible in the classroom, able to change topics to follow a tangent if one arises, sometimes jumping ahead to future topics while answering student questions. Having the preprinted slides out ahead of time would restrict my ability to do this ("That isn't in our handout!"), but by posting my slides after the lecture, students have only one clean file to look over.
4) The students would not have to take notes or be active participants in lecture, since they had all the text in front of them. It's my opinion that being forced to take notes helps student recall and attentiveness.
5) One way instructors get around the lack of note-taking with preprinted slides is to remove critical elements from each slide (keywords, etc.). I find this strategy to be annoying, as it turns the lecture into a game of "find the term", where instead of paying attention to the content, students are focused on finding the missing words.
6) I have a number of in-class participation questions embedded in my slides, designed to check to see if students are understanding the content. If the students had these available during the lecture they could easily sketch out the answer to the questions while I was discussing content related to the questions, defeating much of the purpose of the questions (which is often to get students to think back, remember, and apply content we've just covered). An obvious solution to this would be to remove the question-containing slides before posting, although then the students wouldn't have the questions available afterward.
7) It's a minor point, but students would try to take notes on the slides, even though there probably wouldn't be enough room for detailed note-taking.
There is, however, one large benefit of giving out lecture slides ahead of time: the students don't have to frantically redraw complex figures to take notes related to the figures. Thus, if I know there are complex figures that I'll be spending a lot of time on in lecture, I post these as PDFs on my website before the lecture.