#1 Definetely worth the money. It'll improve your fishing tremendously.
#2 I have a HB 898 si. It's great, but if I had to do it over again, I'd keep saving and buy one of the bigger screens.
#3 For me, the SI is for finding structure and cover. You'll find dozens of places to fish the first day you use it (I found 148, new to me, brushpiles in 1 day on a lake I had fished for years). You will learn to see schools of fish fairly quickly and identifying individual fish will come as well. There is a lot to be said for adjusting the unit to see the fish. You'll learn, it just takes turning on the unit and playing with it. You can freeze the screen and then make adjustments to the unit. This technique lets you see how each adjustment changes what you see. You can also use the switch fire mode as a regular fish finder. One thing that helped me was to print out the manual (it comes on a cd) and carry it to the lake.