The Tamron 28-75/2.8 was the first lens I bought -- actually ordered my DS along with this lens, and it's always been my goto "people" lens, as I find the FL range about perfect for the purpose.
The FA28-70/4 should easily The extra stop of max aperture was a necessity for darker venues, but the greater low light sensitivity of later model Pentax bodies probably makes up for this, and the cleaner sensors at higher ISO also compensates from an exposure standpoint.
One of the reasons I chose the Tamron was that it was considerably smaller than the other f2.8 lenses in this range as it used aspheric lens elements to reduce the front element to 67mm compared with the 77mm that's more normal for a lens of this spec. I felt that it would be more pocketable, and would be less intimidating to the people I shot. This has turned out well, and the f4 Pentax should give you the same qualities.
Congrats on the lens. Sounds like you got a very good lens at a very low price!