A decent Style
Even though the song selection isn't the best, the song difficulty and game modes available (Beginner Mode, Drill Mode, Survival Mode) make 5th suited for beginners as well as for experienced players.

Simply the worst IIDX in the series
Most fans agree this has some of the weakest song selections and by far the worst innerface ever. Its innerface is so bad its the only one thats only been used on one style.

Excellent version
Personally, I enjoy the songlist in 6th Style better, but this is definitely the version to start with. 5th Style, has by far, the easiest songlist. Don't let that discourage you, there are plenty of difficult songs on there, but the difficulty curve compared to other styles is significantly less. The first style with the addition of Beginner mode, makes the difficulty transition that much easier. The official controller is a must.