I've seen them, and I thought they were pretty decent. There is a large variation in quality, but they're generally pretty interesting. I actually liked the pseudo-documentary parts (Origins I and II) - it was an interesting look at the world of Halo and its history. (Then again, I like history in general, and I find documentaries decently interesting, so you could take that with a grain of salt)
The Duel was fairly interesting, but the animation style is very off-putting. If you can get past the animation, it's pretty decent. It's the only one of the shorts to really focus on the Covenant and the Elites. The next one, Homecoming, was probably one of my favorites. It delves deeper into the history of the Spartan II's, and now they came to be Spartans. This one might be less enjoyable to those who don't already know the general gist of how the program started and how the Spartans developed, but even then, it's still a very emotional and poignant short. I showed this one to several of my friends (who know nothing about Halo whatsoever) and the majority of them enjoyed it. They were a bit confused about some parts, but that is understandable. Halo Legends is not supposed to be a standalone series - you're expected to know something about the series to get the full effect out of it.
The fifth short (Odd One Out), the one made by Toei, may be the only point I agree with the reviewer on. It was light, hilarious, and just overall awesome. The director of the short is the same director who directed Dragonball and Dragonball Z - it has very similar humor. (and a similar art style) It's not canon to the Halo universe, so you don't really need to know much about Halo to enjoy this one. Besides, how could you not like a guy whose official designation is 1337?
The sixth short (Prototype) was also very good. In my opinion, it had the best animation out of all of the shorts. After the 4th and 5th shorts, it's probably my third favorite. It doesn't require a great amount of knowledge of the Halo universe, but knowing something about it makes it more enjoyable.
I actually don't remember much about the 7th short, other than the fact that it had decent animation... it wasn't one of the better ones, at least in my opinion.
The eighth short (The Package) may have been my least favorite - the fully CG artstyle didn't really impress me, and the plot of the short wasn't that great.
In order of quality, shorts 4,5, and 6 were very good. The first two (the Origins) shorts were decently interesting, but it may not be something you want to show to someone who knows absolutely nothing about Halo - it may just bore them. The Duel was fairly good, but I can see the artstyle alienating a lot of people. (including me... I had kind of a hard time watching it because of the whole "filtered stained glass look")