I have the Domino, the Mayfel DD40 and of course the Lamello Biscuiter...and I love them all. They all have their unique quirks that get you out of holes. The Domino's ability to create a mortice just saves time, both in setup an operation (as opposed to using a router and jig) and is worth the cost just for that. The Lamello Biscuiter is just so easy to use, especially with their range of extras, such as hinges and connectors. The DD40 is great. It does what it says on the tin and is a breeze to use - it drills holes from 5mm, 8mm, 10mm (all of which I have) and 12mm (which I haven't) - I use it for 5mm shelf pins (the DD40 cuts dowels on 32mm centres) and for accurate minifx placement (you have to make a jig for the 15mm Forstner bit, much cheaper than commercial jigs). I'm also looking at utilising the Festool FS 2424 Guide Rail ( I have the rail, but not the LR32 System - can't afford it just yet).
My main beef with DD40 is to do with referencing - both the Festool and Lamello use the base, so aligning shelving (providing you get the orientation right) is a breeze. However, the DD40 has a weird base and you can't do this - this IMHO is it's major drawback - they provide 32mm guide rail - good but more cost!!! - have a look and you will see what I'm on about.
In conclusion, I reach for the Domino and Lamello around the same amount of time, whilst the DD40 gets used only occasionally - but usually to get me out of a hole. I would not be without any of the three.