The hotels, car rentals, etc. do tend to be poor value per mile.

That said, while many people use and focus on the values listed above, the question is really going to be what's worth it for you. Many will say they get a great value of their miles by flying long-haul in F at 8 cents/mile (or whatever) vs. the cost of a ticket, however, wouldn't ever actually pay for F, only fly it as a redemption. Which in that case, is that really the value you're getting if you wouldn't have taken the trip otherwise, or would have flown in Y instead?

My point is that while monetary value can be a good guide, its not necessarily the only one. If you're want to use it to splurge for a nice hotel for your honeymoon, 15th anniversary, 40th birthday, or whatever, and its something that you wouldn't/couldn't pay for on your own, and you think the experience is worth it, then it might be worth it for you. In that case, the value goes beyond the cents/mile. Just a little food for thought.