Why I like WSPR

 These are some of the reasons I like WSPR mode:

• About 100 stations (or more) can co-habit in a 200Hz wide WSPR sub-band on each band, time and frequency sharing. In reality, usually far fewer stations on worldwide per band.
• Low power is all you need 1mW to 5W is quite fine. I tend to use 1-2W, but have been spotted at DX distances with a few mW. Some even use uW power levels!
• If there is an opening everyone has a fair chance (anyone can be spotted at great ranges)
• Simple antennas work. No need for beams on HF.
• Can monitor activity (your own and that of others) in the lounge by visiting website www.WSPRnet.org on any PC or tablet.
• Speaking not needed (saves my stroke damaged voice).
• Highly sensitive (12-14dB better than 12wpm CW?).
• Can do other things whilst WSPRing.
• Automatic - requires no manual intervention once running.
• Ideal for accurate comparative measurements (e.g antennas, rigs).
• Ideal for propagation experiments.  
• About QRSS10 equivalent.
• Works from VLF to UHF (watch stability on higher bands).
• A QRP man's dream mode.
• For 2-way QSOs use JT9-1 (about 2dB worse than WSPR).

Disadvantages:

• Needs accurate timing (usually sync'd to internet time).
• Need good frequency accuracy.
• Usually needs a PC (not always e.g Hans Summers Ultimate 3 and similar kits).
• Needs careful interfacing to rig to avoid loops and 50, 100Hz sidebands.

Download WSPR software at http://physics.princeton.edu/pulsar/K1JT/wspr.html .