I have been attending SPSP since my sophomore year, and so this will be my third year attending (although this will be my second year presenting). This conference is really great to mingle with a variety of researchers, both senior and junior, especially if you plan accordingly to your interests.
However, some of the graduate students in my department have warned me about the undergrads who swarm SPSP (particularly those not presenting) to "stalk" (her words, not mine) professors to whom they've applied. This got me really nervous because I have been to several conferences where I have met most of the professors who I have recently applied to and I know I will definitely run into them here (e.g., one of my professors is in the same symposium as one of my top choice PI's). In fact, one even has a poster a couple slots away from mine and I will actually be meeting up with the grad student of my top choice program (the PI wasn't able to make it). I guess you just have to be careful and walk a fine line between being intellectual and interested rather than the annoying undergrad who is trying to brown-nose their way into a program.
Although on the upside, I was also told some programs don't even look at applications until after SPSP so it could work in your favor if you happen to bump into your favorite professor :-)