I have been using Agenda for some time and have now bought the full version. Not so much because I absolutely need it to use Agenda on a daily basis, but because I find the pricing system very fair and want to support the developers.
There are a few things that I still miss in Agenda, and I would like to suggest them.
For one thing, I think a tag overview would be very helpful, so that you can simply drag saved tags into a note. I always use the same tags across programmes, so having them in front of me is helpful.
It would also be good if you could do an intelligent search based on these tags (e.g. #Meeting) or according to the GTD method (@Telephone). Regarding this, a link to the address book would be nice, so that linked contacts can be called up directly and you can send a mail or make a call. Since Agenda is linked to the calendar, this should not be a big problem.
The second suggestion is the possibility to replicate notes. That is, not to duplicate, but to create a reference in another project that updates with the original. This is an option that I really appreciate in Devonthink. Like many users here, I have different projects that often intertwine or points from different projects are discussed in a meeting. A replicant of a file would be a helpful way to avoid duplicate entries and extra work.
Otherwise, the workflow takes a bit of getting used to, but I increasingly enjoy working with Agenda and, along with 2Do and Devonthink, now use it as my most important tool in project work.