Rather than review all the phases, I figure it's best to do the first since you'll be starting off with this one anyway :)
The 'Final Fantasy' direct game series (meaning 1-11) has always been the basis and the best. The spin-off games have always been recieved with mixed opinions, since they can be quite different.
However, the Final Fantasy Animation have always been recieved very poorly. The first anime called "Final Fantasy: Legend of the Crystals" was in 4 parts and took place after the FF5 video game...the translation of names was inconsistance w/ the later released US video game, needless to say it was pretty bad. We all remeber Sony's attempt to show off the PS2 CG movie...which was terrible.
However, FFU is actually the 3rd and best atempt for an animated FF series. As the games it is a completely new story, but with classic FF elements (Chocobos, Summon Spells, Charcters leave & join the party) so in this aspect it is pretty good.
The English dubbing is also pretty good, though Yu's voice actor always sounds like he reading his lines and they should have used someone else in my opinion (that's quite annoying!).
The story will be enjoyed by anyone who enjoys the games, I would base the character types (like Fungus and other weird one) on a FF6 & 9 level = NOT for everyone as some are silly, I do not believe Amano created them, but that's the overall flavor.
This is somewhat like Xenogears animation (mix of CG & Cartoon) but overall not quite that much...but again you get the idea. 90% is animated with only a few scenes and small fractions with some CG special effects like Kaze's Demon Gun (AKA Magun in US).
Overall don't expect this to be based directly on a FF game. I reccomend it for die-hard Squaresoft/FF fans.
The Japanese version would fir nicely in the "Teen" rating, but sadly the US version is slightly toned down and thus younger children would enjoy it :(