I have run Humminbirds since the Super-sixty flasher (!) and like them. The 859ci does not have the side-scan function of the 859si, which makes transducer mounting easier. The unit will work fine right out of the box, but you can customize it a bit to fit your needs. The split-screen options are very handy to see maps and bottom contours at the same time. The internal maps on the 859 are very good, with bottom contours, etc. Put an SD card in and you can record what's on the screen an play it back on your laptop/PC. With the Ethernet port, you can also do plug-and-play radar - Humminbird has a neat 12" HD radar that you can get if you want it.
Are you using a transom-mount transducer? If so, make sure it's in a place with a clean water flow off the hull to get best readings at higher speeds.
These are popular units and the gimbal mount makes it easy to remove. My advice is to remove it when your boat is not in use - otherwise someone else may do that for you.