I wouldn't necessarily say that it's a 'good' buy, as they pretty spawn naturally at every place you have to pick it up from, with the exception of one or 2 places. I'd say the tank is the better buy, since it's much harder to acquire. Although, the tank is just a trolling tool, you get bored of it quickly unless you suck at PvP.
And the tank isn't useful on missions either.
The Buzzard is extremely useful, but like I said it makes the mission too easy. I get annoyed when I join The Los Santos Connection on hard, expecting a thrill, and two lamers go rushing for their Pegasus Buzzards.
Yeah it sucks on missions, but I don't think the buzzard has much of a purpose on a mission either, since 95% of the missions require you to deliver something, you can't really use the rockets unless you want to fail it. The only time I need a helicopter is when the mission takes place on the other side of the map. But I just request a helicopter pickup and kill the pilot. It's not like the Maverick is slow either, and it's delivered to me.