I've downloaded and played around with FamilySearch Indexer, and I must report that it's really impressive. Initially I was dismayed by the idea of collaborative transcription on a thick (desktop!) client, but my trial run showed me why it's necessary for their problem domain. Simply put, FSI is designed for forms, not freeform manuscripts like letters.
The problem transcribing forms is the structured nature of the handwritten data. An early 20th century letter written in a clear hand may be accurately represented by a smallish chunk of ASCII text. The only structures to encode are paragraph and page breaks. Printed census or military forms are different — you don't want to require your transcribes to type out "First Name" a thousand times, but how else can you represent the types of data recorded by the original writer?
FamilySearch Indexer turns this problem into an advantage. They've realized that scanned forms are a huge corpus of identically formatted data, so it pays off to come up with an identical structure for the transcription. For the 1900 Virginia census form I tested, someone's gone through the trouble to enter the metadata describing every single field for a fifty-line, twentyish-column page. The image appears in the top frame of the application and you enter the transcription in a spreadsheet-style app in the bottom-left frame. As you tab between fields, context-sensitive help information appears in the bottom-right frame, explaining interpretive problems the scribe might encounter. Auto-completion provides further context for interpretive difficulties (yes, "Hired Hand" is a valid option for the "Relationship" field).
Whoever entered the field metadata also recorded where, on the standard scanned image, that field was located. As you tab through the fields to transcribe, the corresponding field on the manuscript image is highlighted. It's hard to overstate how cool this is. It obviously required a vast amount of work to set up, but if you divide that by the number of pages in the Virginia census and subtract the massive amount of volunteer time it saves, I'd be astonished if wasn't worth the effort.
I have yet to see how FamilySearch Indexer handles freeform manuscript data. Their site mentions a Freedmans' letters project, but I haven't been able to access it through the software. Perhaps they've crafted a separate interface for letters, but based on how specialized their form-transcribing software is, I'm afraid they'd be as poor a fit for diaries as FromThePage would be for census forms.