I suppose that depends on what you mean by heavy sprint. 3-months long is excessive and burnt me out, but 1 month is not so bad. I totally disagree that it should be a once in a lifetime of a company type event to do a month long sprint. Many companies have a couple of major events a year that present an opportunity to make a big splash. You either take advantage of them, or let others pass you by.

It can kill morale depending on how it's treated, and how the company culture rewards. Just because you choose to meet a goal as a team that requires more than average number of hours put in for 4 weeks does not mean everybody is being dragged along. What does upset the folks doing the heavy lifting is when requirements get added at the last minute with "oh, can we please please just have this one more thing? It would be so awesome" from upper management. That's pure bullshit, and tends to rip apart morale above anything else.