I think it depends where you are, what you're looking for, and how you are with your finances. (ie. What your budget is compared to what you need/looking for.)
Personally, I think that this level of technology/equipment is worth ~$1000... it's an innovative product available to consumers that picks up live traffic, decodes it and plots it nicely for you. If you're running a flight school, this could be a very cheap and invaluable tool. If you're new to aviation, you have to consider if you have $1000 to spend.
If you're in Europe, many (most?) aircraft are equipped and transmit with the required MODE-S/ADS-B. In Canada however, I know that the largest airline, Air Canada does not have aircraft that transmit MODE-S/ADS-B. So, if you're in Canada hoping to see any Air Canada flights, you're sore out of luck. You'll probably be able to see any oceanic flights, and maybe some US-based airlines, but I have absolutely no clue about those.
Hope this helps a little...
