I've used the Psion calendar app before and while I thought it was nice, what are you missing about it?
The one stand-out feature of Psion Agenda, which I've never seen since in any other calendar app (but which appears to be in the Planet Computers Gemini Agenda app, judging by the screenshots I've seen), is the year-planner-style Year view.
Here, one year is shown in its entirety, presented as 12 rows with one month per row, showing every day of the month as adjacent cells. The rows are staggered sideways so that the days of the week align correctly in columns. Each cell is big enough to have a coloured spot to indicate one or more calendar events for that day (or a birthday etc.), and multi-day events could be shown by a bar extending across days. (Of course, given its high-res colour display, the Gemini could do this much better than the Psions were able to.)
Anyway, this is the one major feature that I've missed in every single calendar app I've ever used since having the Psion. It's what I'd call a 'year planner' view and it's unbelievably useful. I just cannot understand why no other software has ever implemented this since Psion did it so well and made it clear how extremely useful it is.
The aCalendar app does indeed look quite nice, as yet another pretty standard calendar app… but it doesn't offer the Psion-style year-planner view that I really want. So I'll continue to wait for Planet's Gemini Agenda app and hope it's good.