...as compared to the typical 4 string bass. Mostly for reasons of cost, playability, etc. I was thinking about getting a 6'er until I realized that I would almost never use the extra strings. So what are your thoughts?
Obviously this answer will vary from person to person, but you're asking everyone. So, here's my answer.
I played 4 string for 20 years. At that point I had Hipshot D-Tuners (Bass Xtenders) on all of my fours because I often needed notes below low E. In Y2K I finally decided to try a fiver, and quickly realized that I could adapt to it, so I bought one. It took a couple months before I was completely comfortable, but I knew this was the right bass for me. I wound up selling all my fours and now I play five string exclusively.
I mostly use the low notes for modern rock, but since Y2K I have played MANY covers that require notes below low E. That includes bass guitar lines in modern country and blues, as well as synth bass lines in pop. Last but not least, the extra wiggle room is great to have when playing in "jazz keys": Eb, Bb, Ab.
I was originally interested in five because of the low notes, but learned that the bigger advantage was having greater range in each fingering position. That applies above the fifth fret, where all notes are in four-string land.
I've considered a six string, but I rarely solo, and have realized that I do not want a bass that has either a wider fingerboard or narrower string spacing. Five string is perfect for me.
I always recommend five string to beginner musicians because the additional range will make them more marketable... that is, it's easy to hit those lower notes without the inconvenience of downtuning. But there's nothing wrong with starting on four or six or whatever either.