Anyone use one? Likes and dislikes?
Yes, and I like it now that I don't use the original mounting bracket.

What model do you use?
Nashbar Elite

Suggested improvements to that design?
I took the handlebar mount and made a makeshift decaleur to mount the bag to my front platform rack.

I'm thinking about trying to make my own so would like to know what works for you and what doesn't and why.
If you're mounting it to the handlebars, keep the size pretty small. Use a hanging attachment and only use it for tools and some gels or light items. Stiff sided large bags (300 cu.in. and up) which mount to the handlebars all have the same problems:
- They bounce around like crazy.
- They sit too high and mess up the front-end handling.
- They have too low a weight limit (limited by the mounting bracket.)

Mounting the bag to a front rack (Nashbar $7 rack, Nitto M-12, etc.) does a number of things:
- Stops the bouncing because the bag is strapped down at the base
- Lowers the center of gravity and improves handling
- Allows you to carry more weight/larger bag

Look at the size of the Berthoud and Ostrich front platform "handlebar" bags: 700 cubic inches! That's bigger than a lot of rack trunks. Check out the serious randonneur bikes, and you'll notice that the old-school platform-mounted bag is a favourite choice among the long distance riders.
The only thing I wish was different about the Ostrich and Berthoud bags: Why the heck can't they make 'em open towards the front? The flap and closure is at the front of the bag and it opens towards the rider... How the heck does that make sense for getting at your stuff while riding? My Nashbar bag is hinged at the front and zippered in the back so I can just reach on in and grab a gel or an orange wedge while on the go.