Recommending that you upgrade from your previous 400t would be a hard thing for us to do. Yes you get the camera and a better screen, but the core functionality here is the same and both units are fairly expensive.
If, however, you've been put off from signing up because of worries of the screen on the 400t then this is a good time to jump in.
The addition of the camera is a logical one rather than a must have and the new improved waypoint and route counts are likely to only appeal to a hardcore set (you know who you are).
The only gripe we would have is the cost. Like the 400t, the 550t is expensive, £468 ($600) expensive, meaning this is one for the regular geocacher rather than someone who would do it on a "I'm bored this weekend", kind of moment. That said, for the price you do reduce your hassle and need to understand technology to a minimum.
In our 400t review we winged that the screen was too dull. Garmin have fixed that so it only seem far for us to up the score from an 8 to a 9 for this model.
