I have been playing acoustic guitar for 5 years and am now considering buying a banjo. How similar is playing the banjo to playing the guitar?
Depends on what sort of banjo your talking about. A 6-string banjo is usually tuned like a guitar, and is as such very close. The difference is that any banjo has a different attack and much less sustain, consequently it is has to be played slightly differently to "make sense" musically.
The four and five string banjos are tuned differently. The four string is mostly considered a rhythm instrument – mostly used in older style music like swing (trad.) jazz, dixieland and dance hall music. The four string banjo is also sometimes tuned like the top four strings on the guitar.
The five string banjo is well known lead and rhythm instrument in country music, bluegrass etc. This banjo is usually played with thumb- and finger picks.
You also have an 8-string mandolin banjo, called banjolin. Finally, you have the banjo ukulele. As the name implies, it is tuned and played like an ukulele.