It's a great game. My 2nd or 3rd favorite Bemani game.
The gameplay is the same as other Bemani, when the notes hit the line at the end of the screen, you hit the corresponding button on the guitar (red blue and green). When the wailing icon hits the line, you tilt up the guitar for a bonus.
Right now all I have is 1st Mix and 2nd Mix. 1st Mix has a very small songlist (25 songs total, including the same song with a different difficulty like of those 25 songs, like Hypnotica on Normal is counted as 1 song and Hypnotica on Expert is considered a different song) and it takes a while to fully unlock all the songs.
I haven't played 2nd Mix yet since I wanted to wait until I got bored with 1st Mix.
Keep in mind that there are only 4 home versions of the game (1st and 2nd Mix for PS1, 3rd and 4th Mix for PS2) even though in the arcade they are up to 10th mix.
With 3rd and 4th Mix, both also have Drum Mania games with them too which is a nice bonus (Guitar 3rd has Drum Mania 2nd Mix and Guitar 4th has Drum Mania 3rd Mix).
If you can get it at a good price, I would try to get the official Konami controller.
However, if you just want to see if you really like the game before you buy an official controller or you think you will be happy with a 3rd party controller, a cheap 3rd party one might be good enough (I have never used a 3rd party guitar so I can't say for sure how good or bad it is)
It took me a year of searching on Ebay but I finally saw a buy it now on the Konami Guitar and 1st Mix for $65 with shipping and bought it (the guitar alone can reach $100+ even used which I think is way too much money) .