Spiros are an old favorite for jazz and other genres and has been a popular string for many years. They were one of the first decent metal strings to be adopted by strictly gut string players-- in fact, lots and lots of us played G & D guts and A & E Spirocores, since the larger gut strings of the mid-20th century still were too indefinite and flabby for good jazz articulation.
They have great sustain and a big sound good for many styles, but remain a jazz favorite. You'll also find them on lots of general use basses, for bluegrass, blues, and other styles where that big sound and sustain are prized. While they are certainly used by many fine arco players, they have a reputation for a scratchy sort of response under the bow -- and their stiffness requires an attention to proper technique. So if you bow a lot, and not strictly acoustically (scratchiness is particularly obvious when amplified), consider that reputation. I have used them sucessfully with magnetic pickups.
These strings are the "Red Mitchell" Signature set, which is a set for tuning in 5ths, which is favored by some players. They are tuned in a "cello" tuning - from low to high, the tuning is C-G-D-A.
Note about 3986 and 3985 sets:
The Spirocores are available in many different sizes and tensions. The most popular sets are the S42 variant; they are designed to be used on 3/4 and 4/4 size basses. If you have Spiros on your bass, chances are very good that they are the S42/S42W set. However, Thomastik also makes a 3/4 specific set, designated the 3885, which is very specifically sized and designed for optimal tension on a 3/4 size bass. For the Fifths Tuning sets, the (yes, confusing) numbering system is 3986=S42 (4/4 and 3/4) and 3985=3885 (3/4-specific).
Don't know which one to get? Get the 3986 variant -- they are, far and away, the most popular (and what you'll end up with if you buy from a store that doesn't specify). Only very particular players bother with the 3985, as the difference is quite minimal to most of us.