I have the BBE 442 and Aphex 204, as well as the BBE Direct X plug in. I've used them for various purposes, and I have used all of them at one time or another for mxing/mastering.
The BBE stuff is great, and so is the 204. They each have a slighly different sound. To my ears, the Aphex 204 handles bass mgmt better than the older BBE. Sonically, it reminds me of the aural exciter features in the TC Finalizer. The BBE's seem to have a smoother top end. That doesn't mean the Aphex has an unpleasant top end. Unlike the BBE's, it is tunable for both the bass mgmt and mid/hi frequency range you wish to center the processing on. The 204 seems to be a more flexible processor, which means you can easily muck up your audio if you have less than stellar judgement.
One place where they have a purpose is to balance the presence of say a guitar track against a lot of digital synth or sampler tracks in a mix. If you use one on individual tracks, be careful using it on a mix or master, becasue it will be double processing the signal.
No matter which type you use, remember that less is more. Overdoing it with an aural exciter creates mix which is hyped in the bottom end and top end. It will sound unnatural and 'artificial'.