Aye gregwils,
Wear characteristics of K-liners are every bit as good, if not better than ordinary phosphor bronze valve guides, due in part to the oil retention grooves within the bore. Many commerecial vehicle engine re-conditioners use them because of their known service longevity and ease of renewal.
Whilst you may be advised that you are able to have closer running tolerances between a K-liner valve guide liner and the valve stem, this can lead to scuffing and premature wear between them. This was a problem that I had with my '74 450 MKIII head guide liners after they were installed by a widely advertised cylinder head "specialist" in the UK. I would not advise there services.
It would appear that whilst the valve guides had been lined, genuine K-liners may not have been used and some guides and liners are manufactured with a high nickel content for economy of manufacture.
CAVEAT EMPTOR! And the measured running tolerances in my cylinder head were "tight" (0.0015").
So, my local engineering shop owner, who runs a very nice BSA Rocket Gold Star and does cylinder head work on car rallying engines, accepted the job of removing the old guide liners, re-boring the valve guides perpendicular to the valve seat to accept genuine K-liners and after installing and trimming the liners, they were lightly honed to give a running tolerance of 0.002" between the guide and the valve stem. The valve seats were then lightly kissed to achieve a good seal between the valve head and seat, this being vacuum checked and found to be perfect.
I installed the valves myself and used the 'Viton' guide seals. So far, after over 10,000 miles use, I have not had any problems and there is no blue haze on the overun.
If you should want more information about the K-liner valve guide liners then I attach a link for information (this is not my local engineering shop):
K-line_articleGood health, Bill