Retribution is a fairly decent expansion pack for Starcraft which comes with a new campaign and new multiplayer missions
The storyline, while set in the starcraft universe, is not directly related to the events of the original Starcraft so even if you don't remember the storyline of the original game you'll still get an idea of what is going on here: The plot is about an ancient artifact called the Argus Stone, presumably of Xel'naga origin, which has been found on the terran world of Aridas. All three races converge upon the planet in order to acquire the artifact, as it is believed that whoever controls the stone will be able to control the universe. The first campaign is the Protoss one, with the Terran and the Zerg ones coming as the second and third respectively.
The campaign comes with actual voice WAV files in the briefings (and missions) like in the original game. While they're not as good as the original ones they work well and actually sound like they came from an unit from that campaign. The briefings do sometimes get repetitive as you mostly get briefed by only one unit and nothing in particular happens unlike the original game.
Regarding the gameplay, the campaigns are very hard and much harder than the original Starcraft, fortunately though they don't manage to be always repetitive. While often you will just be sent in a battlefield which is literally full of enemies, some missions require you to send some units to a specific location in the map, and of course that is not so easy either. The amount of enemies is big, for example in the first Zerg mission the main objective is to destroy 12 battlecruisers that are stored in a very well defended base full of siege tanks and goliaths. Another thing that should be noted is that since the beginning of all campaigns, all units are available. For example since the first Zerg mission you are able to create Ultralisks. And this is a good thing given the high number of enemies in the maps.
As for the multiplayer maps, I did not check out all of them but judging from the ones I played they were fairly well decorated and also decent gameplay-wise. Unfortunately though since this expansion is for the original Starcraft, the maps only use the themes available in the original game and not the ones introduced in Broodwar, making them more repetitive. But overall, there's really nothing to complain about regarding the multiplayer maps.
So in conclusion, Retribution is not a bad expansion and even if it does not add any new unit or stuff like that, it is good for what it is. If you are a big Starcraft fan and are looking for some challenging single player maps or some new multiplayer maps, then Retribution could be what you're looking for.
EDIT: So after having played it more, both online and in SP, I increaase my vote to 7.8 (I don't know how to do that so I'll just keep it as 7.5). This is because the maps are generally of quality and are balanced in MP. The SP campaign would be great to do in coop and since it's not to hard to modify the maps in order to turn it into coop I'll give it a shot some day.