Using the miles is ALWAYS better than earning them. Mostly because I get to travel with Mrs. Fundi. Basically being a cheapskate, Mrs. Fundi and I get to enjoy much better vacations that we could normally afford because of the savings in airfare.
As mentioned above, Free tickets make fabulous gifts as well.
The only time I feel regret using miles is when I see really cheap fares to where I am going AFTER I have spent the miles.