If Rocket Spanish doesn't tickle your fancy, Fluenz comes in a close second. It also does an excellent job of blending grammar and interactivity so that you actually learn how Spanish works. (Compared to courses like Rosetta Stone which don't teach the actual mechanics of the language — Fluenz seems to have a bit of a vendetta against Rosetta, if you look at their sales material. Claws out!) The biggest selling point for me is the undeniable charm of presenter (and Fluenz founder) Sonia Gil. In each lesson you'll see her popping up to explain the vocabulary and grammar you're learning.
Part of the Fluenz philosophy is to restore the tutor experience to the language learning course, and it works: She's smart, sympathetic, and encouraging, and I want to stick through the course just so I don't let her down! Like Rocket Spanish, Fluenz offers a nice mix of explanation and testing. You end up with well-rounded skills in listening, writing, recall and pronunciation. Fluenz Spanish doesn't pack quite as much into each level as Rocket Spanish does and the price tag is a bit higher, but it also has seriously high production values.
Earlier versions of Fluenz were only available as physical copies (so you had to wait for them to be shipped to you), but they now offer a digital download version. The price is the same. Pros Beautiful to look at.
Engaging presenter puts the material into context. Lessons include tests which ensure you've mastered the material. Teaches grammar, rather than relying on gimmicks (watch out, Rosetta Stone!) Cons No free trial. Hefty download to get started. Harder to 'binge' on material. You need to go through all the exercises. No grammar manual — you may need to purchase a separate book if you want a reference.