One thing a lot of people don't seem to think about when singing the praises of the Gamecube controllers is the button layout.
If you're only planning on downloading NES/Turbografx16 games from the Virtual Console (that only require the use of 2 buttons and the D-Pad), and you also want something that you can use with Gamecube games, then by all means avoid this and get a wired Gamecube pad or Wavebird instead, especially as the Wii Classic Controller isn't compatible with Gamecube games.
However, if you're planning on downloading some decent SNES games (and there are a lot appearing now), then you MUST get this. The games for Virtual Console have been designed with the Classic Controller in mind, that means that the controls have been mapped to where they are expected to be on the Classic Controller. You might think that this doesn't mean much......but you can't optimise the controls yourself on most games, so that means if you're using a Gamecube controller (where the `X' and `Y' buttons are in strange places), then you're in trouble. This is particularly apparent when trying to play games like Super Mario World with the Gamecube controller.....it's a near impossibility. But playing it with the Wii Classic Controller is simplicity itself. When using a Gamecube controller, there are also noticeable difficulties with the button layouts (in my experience) when playing Zelda: Link To The Past and Mystical Ninja as well, whereas I can manage them just fine with the Classic Controller. And don't even get me started about trying to play Streetfighter 2 with a Gamecube controller!!! It makes getting used to Streetfighter 3 on the Dreamcast seem easy!
Also, hopefully in the coming months as decent multiformat games come out, the Wii won't get crippled with silly unnecessary tacked on motion controls that just make conventional games harder to play (as has happened with a lot of the 'shovelware' launch titles). The Classic Controller should come into it's own then. I can see it being very good for Super Smash Bros. Dojo.
All in all, you just need to think about what you need it for before you buy it.