I installed harnesses in my S1 a few weeks ago. Probably just because I thought they looked cool. The harness bar was £117 from eliseparts and I bought the harnesses of eBay for about £120 (pair) IIRC. They are Willans and were fitted to Bell & Coville 160's. I don't have slotted seats so they are bolt in types. I didn't have long enough bolts to retain the seat belts. Not sure where you stick the harnesses if you do retain the seatbelts when the harness are not being used.
Negatives:
Slight PITA doing them up but am getting used to this and can do it in less than 20 secs, so no biggy.
Bit of a PITA when trying to reverse or see out of some junctions. Easy work around for this is to simply loosen the shoulder straps when manouvering then pulling them tight again.
No use for small kids in the car. My 4 yo niece was desperate for a hurl in the car but there was no way they could secure her safely. Not sure about child seats but I think ScottF has had some success on that front
Also a bit of a pain if you have pax's who are clueless on how to do them up, which is probably most people.
Doesn't half crush shirts and suits
Positives:
Look cool
Makes you feel very secure and at one with the car - particularly good under braking and I am sure cornering - if driving fast enough i.e. on track.
On the whole sounds like the -ve's outweigh the +ve's but I like them.
2p (well more like 50p)
Stewart
yes yes yes.... another valid point, guess i should just get a cglock instead! ill get me coat!
if i got that for them and managed to get the ones on ebay for £700 or so i reckon it would be worth the swap. if i got them it might encourage myself to keep the car which will be far cheaper in the long run. 
nothing like you G....yet! 
hopefully lotus will have brought out an aftermarket kit by then. i reckon if i mod mine now it will make the decision easier in 12 months time. 
