LyX has the best system for entering maths into a computer that I've ever used. The combination of (a) understanding standard LaTeX (and amsmath etc) commands, (b) keeping track of grouping for me (no need to manually deal with curly braces) and (c) seeing everything appearing immediately means that I often use this over pen+paper for moderately complex derivations. Even if though I've never really used it for full LaTeX documents, for me it's worth having around just for this.