I went for the standard Mule from an old HAWG and find that more often than not its just right for a big day out, only occassionally is it struggling. I found that the HAWG was pretty much empty more often than I needed more space in the MULE.
You can always get a 'pocket' that straps onto the waist / shoulder band that you can transfer to other rucksacks.
On that note, I find that the hip belt on the MULE is a let down as it attaches with Velcro - although its never come undone it doesn't seem the best idea for a mid size pack, also the 'tails' of the waistbelt (the excess I haven't bothered to trim yet) have an anooying habit of coming out of the elastic.
In terms of backpack design there are much better designs out there, however I think they're better than Dakine and OMM stuff but there are better ones out there like Osprey and maybe Golite or others, however Camelbak are always available cheap somewhere.
The best thing about the MULE is the outer flap that the tool puch is attached to can swallow loads. You could put an exped bag in there to put warm gear in there too.