The UI looks great, way better than most of the other trading platforms I've seen. And I like the fact that it's a trading platform aggregator rather than holding funds directly.
The backtesting feature looks especially valuable. How far back in time can you test strategies with this tool?
The UI being great is actually a huge factor. It gives you a more positive attitude towards trading and make you more interested in what you are doing.
As for the "how far back in time" i don't really know but if it goes far beyond it will be a very helpful feature to.