It is difficult to rate a single because usually it means rating a song or two or three tracks, and frankly, if you are buying this single you are probably already a U2 fan, and a review won't necessarily move you one way or the other.
However, I also find it nice to give some insight for those who don't necessarily know what is here and give the advantages and disadvantages of this single.
First the disadvantages. You are getting a song that is available on a complete album, albeit this version is different (and probably better) than what appears on "POP" but is available on the recent "Best of 1990-2000" compilation. You are only getting three songs, and you could wait for the standard B-Sides album that comes with U2's greatest hits.... One song is not even sung by U2 (Willie Nelson) with a U2 sung version already available on the "Stay..." single.
Ok, the advantages. Contrary to my statement, U2 did not put these songs on the B-Sides compilation. The song "Two Shots of Happy, One Shot of Sad" is perhaps one of the greatest B-Sides of the last 10 years by U2, and it was written for Frank Sinatra's 80th birthday. This song demonstrates the versitility U2 have. "Slow Dancing" with Willie Nelson is actually quite cool and is how the song was meant anyway. It is a country song, and Willie Nelson's voice shows that. Again, U2's diversity shines through. Last of all, Sunday Bloody Sunday here is a live acoustic version that is perhaps the best I've heard... and... Edge sings and performs it solo. Awesome.
This is one of the more essential U2 singles in my opinion.