An optical viewfinder can never have any lag, since it's optical it operates at the speed of light. On the other hand with an optical viewfinder you will not see exactly what you will get in you image file. Exposure, white balance, color and image crop (3:2, 1:1, ...) settings are not visible in the optical viewfinder, but can be visible in the electronic viewfinder (depending on camera model). Also, a electronic viewfinder can display manual focusing aids.
The optical viewfinder of a DSLR has a short blackout when the mirror moves. A electronic viewfinder don't necessarily have this blackout.
You can get all of the benefits of the electronic viewfinder on a camera with optical viewfinder using the back screen, which consumes even more power and is hard to view in bright conditions.
Preference for electronic viewfinder:
- When a smaller camera is required
- Flash photography
- Photographing in low light situations
- Fast action (if there is not blackout)
Preference for optical viewfinder:
- Fast action (because there is no lag)
- When long battery life is required
I personally switched to a camera with electronic viewfinder about 2 years ago, after using SLR and DSLR for approx. 10 years, because of the smaller size and weight.