Also known as speed sensors. They run about $40, sit on the hub and sense the earth's magnetic field to determine movement. Thus they work indoors and you can tell the computer to not look for GPS. Good for trainers. The speed and cadence sensor bundle is about $65.

I would not give up on the Garmin, especially of you do not need a device that displays maps. Yes there are internet posts about problems, but I would hazard a guess that most folks find them reliable. Maybe some add'l research to find out the problem and fixes. I know, I know you should not have to be doing this out of the box, but sometimes stuff just gets wonky. If under warranty, call Garmin tech. support, they'll talk you through a fix.

I only say this as I had 2 initial issues with my Garmin 810 back in April. One resulted in a Garmin call and got the unit working correctly. Then I discovered a faulty SD card on problem 2. I now have 70 rides on this unit with no issues and love it.

Note that when first powering up it wants to be outside to find GPS satellites. It can (AFAIK) sync to the Garmin Connect Mobile app to upload activities as well as download courses, etc... That data then can go to Connect on-line, or Strava and RideWithGPS.

EDIT: DC Rainmaker has great reviews and how-to's. http://www.dcrainmaker.com/

Also give a look at the Wahoo devices. The RFLKT has similar functions to the Garmin 25, can use speed and cadence sensors, ports to Strava and RideWithGPS and is $80.