I will put it nice and simple. Phantasy Star 0 is the true spiritual successor to the original Phantasy Star Online (hereafter known as PSO) that debuted on the Dreamcast and later ported over to the Gamecube, Xbox consoles and the PC version Blue Burst. If you were a fan of PSO, this game will draw you back into those wonderful memories of grinding away hours upon hours of leveling up, finding loot and having fun with your friends. While PS0 does take the battle/beat system of PSO, they have gone and improved on it as well by including PSU's Photo Art system.
While the game is rather fun, especially with the limitations with the DS, even this game has its flaws. Such flaws include not having a quick swap panel system, but is remedied by an alternative method, having only a Level 100 cap like in the original PSO, opposed to ver 2 and Episode I & II editions where the cap was level 200. And most importantly, while it's not Sega's fault; Nintendo's online WFC system. Because of how its handled, you can only match-make with either random people or join up with your friends, (after swapping your friend-codes with them.) And lastly, I'm sure this must be due to the limitations with the DS, but the character creation system isn't as robust as to PSO or even PSU for that matter. There are very few options to choose from when creating your character so expect to see clones running around during online play. :(
If you were a fan of the days of PSO and want to relieve those memories with this new installment, I highly suggest picking this up. While it has its flaws, it's easily overlooked as this game will once again suck your life away as you burn the midnight oil trying to find those red rare items. :)
Hopefully if this game sells well, Sega will make a sequel, and improve on it. (Hint: Sega, increase the level cap to 200!)