Just wanting to know how efficient the 5 inch screen is.  Am I going to be disappointed with the visibility compared to the 899ci HD SI? Should I fork out the extra $300 for the 899?

Yes. The wider the better

The 698ci HD SI Combo has a narrower display than the two units it replaced:

598ci HD SI Combo and 798ci HD SI Combo.

What you do not want with Side Imaging or any side looking imaging product is a narrow screen.

BIGGER is better certainly applies to that technology.

Yeah that's what I figured.  Just how great is it?  Should I mount the screen by the steering wheel or the trolling motor?

How great is what???

The size of displayed subjects with Side Imaging, Side Scan, SideVu, and SideView is based on the range setting.

Unlike the down looking imaging and regular 2D sonar which is based on the depth.

The images scroll from the top of the screen to the bottom of the screen.

With all of them set to the same range, the wider the screen, the larger a subject will be displayed.

For a less costly model, the new Humminbird Helix 5 Side Imaging unit would be a better choice than the 698. Those models are due to be released in December.

Yeah that's what I figured.  Just how great is it?  Should I mount the screen by the steering wheel or the trolling motor?

I added side imaging to my trolling motor this year and I am more than pleased

I use Side Imaging at the console and bow. It is a search tool and a fishing tool.