Pros
For the most part, it's a high volume restaurant. One bad tip or stiff won't ruin your income for the night as at restaurants with more limited seating. It's a 24 hour restaurant so there are a lot of available hours. If you're willing to work overnights, you can often pick up shifts that way.
Cons
Management will make or break you. I've worked for great managers and worked for terrible ones. I worked for a franchise, so the rules were never quite clear; they claimed they followed corporate brand standards, but one look in the brand standard book would should quite a few things that weren't followed. You never knew if you were breaking a rule because the rules changed at least once a month. Denny's tends to have weird rushes when you're not staffed for it. 2 PM on a weekday should be dead, but I've sold a $250 hour by myself at the time in the past (consider that your guest check average is less than $10) The food kind of sucks and you'll get sick of eating at work in a few months.