While I've only used it for batteries, I'm very familiar with my EM3EV frame bag and love it. It's padded, really well-built, thick and LONG velcro straps. The only thing I wish is what I wish for ALL frame bags--that it'd be wider. Even for batteries. I'm about to write to Justin at ebikes.ca to suggest this as a product idea: a wide-load frame bag. An idea just screaming to come into reality. Many of us are squishing bricks in there and not able to close the thing, which also stresses the zipper.
The EM3EV frame bag has internal [thin] velcro straps which can either be useful or get in the way, depending on what you're doing. I try to use them, but maybe their placement could be a bit better [for me, anyway], not 100% sure.
I bought a brand name [forgetting right now--black and orange] frame bag of about the same size off Amazon--really inexpensive, but sucked so bad I deemed it unusable, including being totally unpadded.
Basically, if what you want to carry will FIT inside the EM3EV frame bag, I'd say go for it. Again, specifically width. If you have heavy stuff like bottles/jars (salsa, anyone?), you may end up instinctively putting in something like cardboard supports(?) to keep it more rigid, to maintain its shape. Sometimes you just have to experiment. Let us know! I may move my batts out of my frame and was thinking I might keep the bag there for toting other stuff around. The supplies I find I want on each trip keeps growing, beyond the 'pump and flat repair': a windbreaker, gloves, rag for sweat, spare everything (flashlight/headlamp, batteries), more and more tools, etc. It's starting to get heavy!
Neptronix has the EM3EV bag and a similar one (Falcon?) and says they're very similar but may slightly prefer the Falcon (I don't think he said why in the video), but I was making an order anyway at EM3EV and got it, and I think it was cheaper than the Falcon. Last I looked, EM3EV lowered their price on them and are now a screaming deal. I would not hesistate.
Minor point.. I don't like the rubber monicker/label on the bag and have tried using a Sharpie marker to make it all black. That worked for awhile, but eventually it wore off (open to ideas). I may have to check if it's glued on or if I could remove it with a knife or seam ripper. Though it probably means nothing to any passerby, I want everything black and stealth as possible, and the letters "EV" are (if even subconsciously) suggestive of "electric vehicle", and do draw the eye to the bike.
The only problem I have with EM3EV is, if they still do this/haven't gotten in trouble yet, they must stop skirting PayPal rules by insisting people pay by sending money as a "gift" instead of "goods" which they are goods. PayPal doesn't charge a fee for "gifts", so they are increasing their profit margins by skirting PayPal's rules, and more importantly, depriving you of Buyer Protection. I don't know of any small or large seller who does this for any goods. People just need to refuse, and pay normally and let EM3EV have that discussion with PayPal if they wish to be bold & press the point. They could simply raise their prices that slight bit, and be on the up-and-up. I think it would also be legit to raise prices and offer a discount for non-paypal methods. I'd personally be fine with that. But to force or insist customers submit payment as a "gift" (when it is not a gift), and deprive them of transaction protection in the process, that is not legit. And is, in factual fact, shady business practice, which they are anything but in other respects, so it especially does not look good on them. They are already undercutting the domestic sellers by being based in China, so they are generally the price leader, and I don't see any comparable (and more expensive) domestic sellers doing similar shady skirting of payment rules, so I don't see why the rules should apply to everyone but them, and why it's even such a big deal to them that they'd jeapardize their access to such an important payment system. (Even if that system is politically repugnant.)