Hi Gonzo,
If you want the best gains I think it is worth it. I luckily bought mine second hand from Paul (LickyPaul???) who has sold his Teg now. I think it's the only mod he put on to his and he dyno'ed the result at Thor Race Engineering. I can't remember if he posted a scan of the result but I have a copy I can scan if you want to see it. Unfortunately, one of the graph colours is yellow so, I'm not sure how well it will scan in.
Simon - Do you have this dyno on your file sharing site?
The result was 9BHP and 6ft/lb gain and a very low and consistent intake temperature. Not bad considering no other mod.
The sound it makes depends on what exhaust you've got and if your car has a stock ECU or mapped. I don't consider it particularly loud and I'm quite fussy about things like that.
I still think the Injen is good value and looking at what Simon has done with a 5Zigen Ram air feed in to a stock box with a more free flowing filter (like the Spoon drop-in) you can get some good results too.
Whatever you do on this intake will determine how much you'll gain with the other mods you do later on. It's one of the most important things for NA.
Cheers,
Jon