I have the Edge Insight CTS and like it, found it worthwhile just for curiousity sake and a better read-out on some of the displays. For example one of the options I always keep on display is the fuel gauge which is shown in %. I've found this to be very accurate as opposed to the needle gauge on the dash for seeing exactly how much fuel you have (it's quite a bit more than you think). It does allow you to view quite a few other displays that aren't available through the EVIC. I don't think it dims "with" the dash lights, but it does dim on it's own as it gets dark. It is totally plug and play. As for seeing the trans temp, my 2011 showed this on the EVIC so I chose not to use a spot on the CTS for this. Not sure about your 2008 but you may find once you install the CTS that trans temp is viewable without any further addition.
Since my truck is a Canadian Longhorn (kilometers per hour) I find the MPH impossible to read on the speedometer so MPH is also one of the displays I leave up. Boost, gear, post-turbo EGT and DPF temp are some of the other displays I always leave up. Some will say it's not worth it but I like it and I'm glad I added it. I think it says on Geno's website everything you'll be able to see... just too bad Ram didn't let us see these on the EVIC.