Depends on what you plan to use it for. If you're planning on keeping a ton of media on there, you're going to run out of SSD space really fast and you'll need to go external. Otherwise, it's a fantastic machine and well worth the price.
Since the MBP Retina comes with SSD stock...buying a small USB portable driver would be the most cost effective way to have increased storage on the go.
Spend that ridiculous SSD upgrade money from Apple on upping your RAM (like Karen suggested).