I've got a Humminbird 798 CI SI with the XTM-9-SI-180T transducer that came with the unit. I've been reading about another transducer to upgrade the 798 unit...XTM-9-HDSI-180T. I'd like to hear if anyone else has:1)upgraded to the HDSI transducer. & 2)if they recommend the new HDSI.

Yes, and YES.

I still use both. I have the Compact SI transducer on the transom and the HDSI transducer on the trolling motor which gives me the opportunity to see the difference in performance.

Some don't see any difference when doing a swap since they don't have both to compare.

The HDSI gives a sharper image because the frequency cyrstals are longer and produce a narrower beam.

I have the 798ci SI also and move it from console to bow and back depending on the water I am fishing.

Doesn't the HD stand for High Def? I would think it would give you a better picture of what you're looking for.

The HD in the HDSI transducer model number means it has 800 khz capability which the 798 does not.

I bought a used 798 unit and it came with the HDSI transducer(Owner had allready done the upgrade). The images are great on it. Since i never had the option of seeing the images with the standard transducer i cannot comment about that other than to say I don't think they could be any better.