I've liked most stem bag/caddy type things I've tried. I got a couple of the Revelate Designs versions years ago and they've held up really well. I tried the REI version because it was cheap - it works OK for the price (especially if you see them on sale) but the materials seem cheaper. I've seen the Randi Jo and Outer Shell versions recommended a lot, and will probably try one of those next. Important features for me are exterior pockets (for phone, granola bar, etc.), one handed cinchy opening, and ability to mount on either side of the stem. Bonus points for bottom webbing/stabilizer strap to run through the fork or front rack.
John in Minnesota