This series takes up where Zeta leaves off, and has some big shoes to fill. Unfortunately, that never happens. Apparently the idea was to make a Gundam series that didn't take itself too seriously. Unlike say, G Gundam, this never really works since ZZ is picking up after one of the most serious Gundam series ever.
The first half is generally regarded as a failure, and is not considered to be worth watching. Suffice it to say, the characters are unlikeable and one-dimensional, particularly the antagonists. The action and writing also feel extremely forced, for example, the first five or six battles are fought with both side's mobile suit chests open, exposing the pilots. This is explained by malfunctions and pilot ineptitude, but comes across as a bizarre way of making the suits more personal.
As far as the one-dimensionality of the characters goes, the bad guys - even the ones from Zeta such as Yazan and Haman - are turned into morons. Captain Bright seems to be fine with allowing a kid to steal his mobile suit. The main opponent of Judau's at the beginning of the series is a campy sort of Zeonic tuxedo mask, obsessed with flowers.
But that's the first half...the second half becomes darker, but has to work with what was served in the first half. If the entire series had been written this way, it would be worth watching. Unfortunately, the damage is done and the characters are unlikeable and the overall plot is convoluted and forced. It doesn't feel like anything in this series really matters in the end, and that may actually be the case: The new Zeta movie, by leaving Kamille alive and well at the end, essentially retcons all of Double Zeta.
If you value your time, don't watch ZZ. If you can handle a really campy, horribly written series, go for it. It is a Gundam show, after all. If you think you have to watch ZZ before watching Char's Counterattack, don't worry. There isn't much to miss.
