Are Keilwerths any good? How is their intonation, build quality, tone, quality compared to makes like Yamaha and Yanagisawa.
Yes.
I have three Keilwerth tenors-- a new Keilwerth SX-90R, a 1980 Couf Superba 1 and a Bundy Special from about 1964. I also have a 1945 silverplated Conn 10M. As Hurling Frootmig notes, do a search, as there is plenty of information.
Tone Quaility -- Keilwerths are more along the lines of a Conn, i.e. a "Vintage American" sound, rather than a "French", i.e. Selmer sound. Yamaha's and I gather Yanis tend to fall more into the brighter, focused Selmer-type sound, rather than the darker, spread sound that typifies the "American Vintage" type of sound.
Build quality -- I have found the build quality to be excellent. Although there has been a fair amount of criticism of the levelness of the tone rings on the "R" models, I have not found this to be a particular problem (nor have Doc Tenney or Randy Jones, who helped me pick out my SX-90R). I have also found Keilwerths to be very rugged, solid horns. As a result, they are a bit heavier than Selmer or Selmer clone saxes.
Intonation -- The intonation on my SX-90R is flat out the best of any horn that I have ever played.
There are a number of top players who use Keilwerths, such as Ernie Watts, Sax Gordon Beadle (Couf S1), Mike Tucker, Ron Holloway, etc. Ultimately, however, it is a personal choice that you have to decide for yourself by trying the horn.