These harps have taken a bit of a bashing lately in this forum. I've replied privately to some inquiries, but now that I've figured out how to *successfully* post a message publicly, here's my take.
First off, the Koch 980 and the Slide Harp (both from Hohner) are hybrids. They have the chromatic slide that raises the pitch a semitone, but the use Richter tuning (like a diatonic) instead of solo tuning (like a regular chromatic). For this reason, I think of them as "semi-chromatic."
Another distinguishing feature of this harp is that valves (none on the Koch, draw notes only on the Slide Harp - the main difference between them) don't prevent diatonic-style bending, and mean fewer valve problems - sticking, buzzing, etc.
Little Walter and Rice Miller both recorded with the Koch 980, and both used it the same way - played crossharp with the slide held in(in Ab on a C harp). They would dip the slide out and back in while playing a note, or jab the slide in as a note approach (the slide dip and the slide jab).
Walter used the 980 on "Oh Baby (You're Gonna Miss Me When I'm Gone)" and on "I Love You So," (an alternate take of Oh Baby). Sonny Boy used it on "Dissatisfied."
Another approach is to play it in G, using the slide to get missing notes. When playing in G, it's awful nice to get an F# on a D chord, or get the missing Bb and C# blue notes in Holes 6 and 7.
There are more cool possibilities unique to the 980, but I'm saving them for an article in HIP.
George Mayhew's advice was to skip the 980 and get a 10-hole - or 12-hole regular chromatic. Two comments there.
First, the tuning is totally different, and so is the reed response. The 980 allows you to keep that all-important G draw chord in the first octave, diatonic-style reed response and bending, while still having the advantages of a slide. This is a unique set of desirable characteristics that the solo-tuned chromatics can't deliver.
If you do want to get a regular chromatic and learn the beauties of that tuning, why mess around with a 10-hole instrument like the 260? True, it's small and therefore a nice pocket harp. But it's bound to frustrate anyone who gets serious, because it lacks the full three octave range present on the regular diatonic, on the Koch 980/Slide Harp, and on the regular 12-hole chromatic.
Remember, the tuning on a regular chromatic is the same in all three (or even four) octaves. That makes it a lot easier to go sailing up into the top register. Even on a regular chromatic, I find myself wanting to take it even higher than that. If you don't like the price of a Hohner 270, get a Hering (Dick Farrell has them for about $50).