I have used the Golfsmith shaft Butt Inserts in each of the new SE600C clubs I built. I wanted to achieve a specific swing weight of D2 for my 5-9 irons and wanted D5 for my 4 wedges: pitching, gap, sand, and lob.
I used hosel weights in these buttend inserts but you can also use tungsten powder to achieve the desired swing weight with a cork plug. I glued the weights inside the buttend insert and glued the buttend insert to the shaft.
If you're serious about swing weighting your clubs, give them a try.