I have not spent any time with Forecasting in recent years, but at the time I found it pretty icky and awkward. Forecasts are appropriate if you’re a multi-tier quota based sales company with potentially hundreds of salespeople and you want to track units or a mix of products (30% of your sales need to be from services, or you need to sell 10 warranties per month). It seemed like a lot of extra work on a sales organization for normal sales tracking. I typically try to use some custom fields on the Opportunities to meet any special reporting requirements before turning on Forecasts. My recommendation is to train salespeople to keep their Opportunities up-to-date and forego Forecasting!