TrainingPeaks is worth it....IF you are actually trying to fitness train, and not just riding your bike. A powermeter is an amazing tool and a great thing to have for training. If you actually use a power meter to train, it will change how you ride and train...otherwise it is just a number.

Even a 'cheap' left-arm-only meter is enough. Most people don't have enough leg imbalance to matter really...although as fatigue sets in your FTP drops, and leg imbalance is generally found to increase.