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 SOLO Tuning Set, which is a set for tuning one step higher than standard E-A-D-G tuning. This is commonly used by arco players who are doing solo repertoire; however, many folks buy solo sets to tune to "standard" tuning to create a sort of "extra light" tension option. They are tuned, from low to high, as F#-B-E-A.
Note about S42S and 3886 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 set. However, Thomastik also makes a 3/4 specific set, which in the solo gauge is designated the 3886, which is very specifically sized and designed for optimal tension on a 3/4 size bass. For the Solo Tuning sets, the (yes, confusing) numbering system is S42S=S42 (4/4 and 3/4) and 3886=3885 (3/4-specific).
Don't know which one to get? Get the S42S 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 3886, as the difference is quite minimal to most of us.