I've tried many brands of flatwound strings and love many of them, but they fade in comparison with Thomastiks!
They're quite a light gauge string and low tension, which isn't to everyone's taste from a flatwound, but they are absolutely lovely under the fingers - smooth to the touch.
Although the feel of them alone is worth the asking price, it's the sound and this is where I feel no other string compares. On fitting them, you still get a fair bit of brightness while they settle in - not roundwound bright, but not the dull thud some people associate with flatwound strings. This in itself sounds great, but it's as the strings break in and mellow out that they really come into their own - rich and warm, full or presence, despite the light gauge. They genuinely sound better the older they get and I had a set on a Precision Bass for 5 years with no need to change them (obviously, I wipe my strings down after playing). They were still on the bass when I sold it!
The good thing, is that they sound great on a Precision and Jazz Bass - I've found that some flats sound better on a Precision (La Bella deep Talking) while some sound better on a Jazz (Rotosound) but I'd happily have Thomastiks on both!
They are more expensive than some strings, but given that you will get years out of them if cared for, they are worth every penny!