<p>I can only offer anecdotal evidence, but FWIW my personal experience corresponds well with what common sense and most other posters seem to indicate:</p>
<p>In a nutshell, it might help, and it certainly can’t hurt.</p>
<p>Frankly, the worst that’ll happen is you get no reply and it makes no difference. But if you’re a credible candidate in the first place, no professor is ever going to think, “Oh, I was just horribly inconvenienced by a polite e-mail from a stranger asking about the work I do. That’s inexcusable, and I’ll go out of my way to make sure he/she is blackballed by the admissions committee.” (And if one actually did, well, that’s not the kind of department you’d want to be in.) The best outcome? Well…</p>
<p>The personal experience: I’d been out of school for a few years when I decided to apply to PhD programs, but I had a clear (if broad) set of interests that I hoped could be refined into a research topic in my field (public policy). I started looking up schools based on the general reputation of their programs, then delved into the faculty directories to see who (if anyone) might have interests similar to mine. The summer before applying, I picked one or two faculty at each prospect school and sent each an e-mail with very brief personal background, a sketch of my interests, and a question or two, including (always) the question, if you’re not the most appropriate faculty member to ask about this kind of research, who might I contact instead? Overall I got a few ignores, one or two very curt replies, a handful of relatively useless boilerplate responses, and quite a few genuinely thoughtful, helpful replies, including some referrals to other faculty. (Some replies were much quicker than others, though; many profs are not diligent about answering e-mail. In each case I allowed myself to send <em>one</em> polite reminder e-mail before giving up.)</p>
<p>I followed up on all the responses as appropriate, had a few interesting ongoing exchanges, revised my list of prospect schools, and then arranged personal visits. This was admittedly expensive and time-consuming – for schools that weren’t within a couple hours of my home, I took a week for a marathon autumn road trip hitting half-a-dozen of them – but IMHO it was invaluable. In each case I scheduled things through the appropriate administrative staff, emphasizing that I was considering applying and had already corresponded with professor such-and-such, and asking insofar as possible to meet in person with both faculty and current PhD students in the program. Some schools were <em>much</em> more helpful with the logistics on this than others… which itself was a revealing bit of information about how the programs were run. Long story short, top faculty (and students) at several schools were remarkably generous with their time, sitting down with me for lengthy conversations about the programs, their work, and my interests.</p>
<p>All this helped me revise the prospect list again before applying, and of course anyplace I’d had a personal contact I was able to reference that in my application materials. In the end I still didn’t get accepted everywhere I thought had seemed like a good fit – there are <em>lots</em> of variables involved, after all – but I <em>did</em> get admitted with full funding at one of my top prospects, where I’m currently studying quite happily, including with the professors I wrote to and spoke with here. I’ve been told that in this field, at least, very few prospective applicants take the trouble to send personalized inquiries to faculty, and almost <em>nobody</em> bothers with in-person visits before applying… and there is no doubt in my mind that those proactive early contacts helped me get to where I am right now.</p>