I have some Motorola DTR550s on order. Can anybody here suggest any sites I should be looking at to learn more about these before the arrive.
This site right here will give you all you need to know. Do a search on the DTR410, 550 or 650. The Product Reviews section also has a good thread. Once you get the radios, ask any specific questions on this site and there are lot of people to help. The host of this forum sells a ton of these DTR radios and they know them quite well.
They can be very confusing at first, with all their capabilities and options, but it is easy to just get them fired up and talking to each other right away. They take Motorola professional-level accessories such as speaker mics and surveillance mics; they have VERY clear audio and have about the best range of any UHF radio.
One cannot rewrite the laws of physics though, and they may be able to outperform any FRS/GMRS radio on the market, but you are also not going to get 5 miles range with them. (Order the longer factory antenna for slightly better range.)
Because they are digital, they will also not transmit if another radio is not within range. Unlike an analog radio that might transmit some barely intelligible bursts when beyond their range, digital radios give you 100% readability or nothing. (Listen for the beep tone before you talk to be sure another radio is in range.)
I now have a half dozen of these radios and love them. I even picked up a DTR410 from our forum hosts for its extra management functions, and I can use it to listen remotely, turn off a radio remotely and do a group time sync.