1) Is it easy to install?
Assuming you have minimal knowledge on how to follow instructions, then yes. Adding on the experience with vBulletin is a plus, obviously.
2) Is it worth the $125?
If you have a mobile-based audience, then yes. If your website/forum deals with mobile-based content or mobile technology, then yes. If you generate revenue from your website via advertisements and simply want to give something back to those niche members who browse your forums with mobile devices, then it's up to you. If you simply want an easier method of browsing your forums wirelessly, then no.
Obviously it doesn't have the latest and greatest features that are introduced with every version of vBulletin. There are also some security features (or lack thereof) that don't fall in line with those found in the parent product.
4) Are we happy with it?
Sure. But the success of the WAP site is the fact that this is the #1 BlackBerry-related website in the world and has a membership following that allows the actual utilization of the product. Without the popularity, the content focus, the massive amounts of user traffic, the ad and sponsor-generated revenue, and our loyal members, having the WAP interface would be pointless (my opinion and not that of the website.. just seems logical).
As mentioned by bfrye above, WiForums is a separate product from vBulletin that connects directly to the vBulletin database and pulls the data into its own frontend. This product should not be confused with a vBulletin hack or add-on, which are quite common in the vB world. While it is a great product, it really only becomes useful for a) MAJOR forums with loads of traffic (someone's primary or secondary business), b) wireless and mobile technology subject matter forums, and c) people who simply have nothing better to spend their money on.
Hope that helps.