My friend just went to a SCCA autocross this past weekend, and he said it is very evident on which car has LSD and which car does not. The one without LSD has a tendency to spin the inside wheel alot when acceleration hard off a sharp turn.
Basically, if you like to hang your car at the limit, then get it. If you don't usually push the car and just casual driving, then I don't see the reason why you need it.
Also, when I got my car with LSD, I had to get the car that is fully loaded, so that is around $1000 or so of premium. I don't know if this limitation apply to where you live, if it doesn't, then I guess $350 or so for LSD is a steal.
I never intended on racing in an autocross.. so I didn't get it. Plus in my area, the LSD and Heated seats come together.. so thats almost a grand. I figure the most I'd do is Street Drag... where LSD really wouldn't play a factor.