It depends. If all you want is a single graphics card, or ethernet card, or sound card then the answer would be no.
If you plan on maxing out your Amiga, then the cost of the Mediator will pretty much pay for itself when you can add cheap ethernet and sound cards (usually around $10-$20USD) and second hand Voodoo cards can usually be found pretty cheap.
Performance wise, the Voodoo card blows the doors of the Zorro alternatives and the 3d chip on the Voodoo3/4/5 allow games like Quake to run in full 3d on 512x384 screen at similar framerates to the pseudo3d clickBoom version of Quake on a 320x240 screen.
As for soundcards - the sb128 is the only true AHI supported card (aside from the Terratec card which I don't have, and is slightly less noisy and has digital outputs, but gives no added performance boost). The quality of the driver is slightly buggy and prone to lockups when playing MP3's, but other than that has given me no grief via AHI. People do complain that the sb128 is rather prone to hiss, however, and I think if you're planning on serious audio work, a Zorro card might be a better choice.
As for Ethernet - since about a year ago, I've had no problems with my setup. Earlier drivers did cause an ethernet lockup, which some users still seem to suffer from on their setups. As I said, for me it works without a hitch.
While a luxury item to most, my TVCard is a great card for me allowing me to get video captures and stills through VHI Studio. Also, I have my A1200's composite out routed into the card as well, and I can view most AGA screens on a window on my WB screen. Video capture can be done through VHIStudio (which you must register, or else an UNREGISTERED text will appear on the capture). VHIStudio is good for getting snapshot stills from video, but it's video capture leave a slight bit to be desired - no sound, and encoding is limited to Quicktime .mov format which is a bit slow to encode (but far faster than earlier versions of VHIStudio which saved in mpg video).
All in all it depends on how you want to expand your Amiga. For me, I wouldn't be able to live without my Mediator, and it's far cheaper than trying to buy seperate Zorro solutions for GFXBoards, Soundcards, ethernet cards altogther.
