First, a good starting point on RTL-SDR is the blog site www.rtl-sdr.com.
(I've mentioned it here before and forwarded some relevant posts about things you can do with an SDR such as the RTL-SDR.)
The RTL-SDRs are almost all the same chinese, ultra cheaply made receivers in the range of $15-25. It isn't so much about which device is trustworthy, it's what tuner chip does it have.
At this point, I suggest getting one with the R820T2 tuner which is newer, seems to perform a little bit better, but costs around $5 more than the others. IF you get an R820T (not T2) that will do fine as well.
Places that "specialize" in RTL-SDR and aren't too overpriced are:
- Nooelec, http://www.nooelec.com/store/, They also sell through Amazon.
Their "NESDR Mini 2 SDR" was mentioned on the net $25. It has the R820T2 tuner chip. It's not clear if they upgraded any of the other components other than the included mini-whip antenna which you will probably replace.
- RTL-SDR.com is also selling their own branded R820T2 usb dongles. They also sell through Amazon. They claim upgraded components and a better oscillator (though not temperature controlled) than the usual ones which can easily have a -75 to +75 PPM error.
At this point, almost all of the devices come with an MCX connector so you will need an MCX adapter to hook it up to a real antenna. Both NooElec and RTL-SDR.com's amazon store have MCX adapters.
Also if you don't have any, get some USB extension cables (A male to A-female) around 10 ft in length. These devices have no shielding, so you want to get them as far away from your computer as you can while sticking within USB length limits.
Try to get a good quality USB extension cable with a ferrite core near the end to cut some of the noise. You might also want to buy some additional snap on ferrite cores.
Note: The frequency range on these is about 24 Mhz - 1,700 Mhz so you won't be able to do any HF with the exception of 10 meters and maybe 12 meters. An upconverter can be used if you want to receive HF.
If you connect the RTL-SDR to an outdoor antenna, keep in mind that almost all of these devices have no viable ESD protection.