There are several of this style of bag around now; Topeak have a range and Lezyne have also introduced them to their portfolio. They all share a similar profile and are designed to fit beneath a saddle and down the seat pillar. I bought this one from SJS Cycles and they advertise it with 2.5 Litre capacity but Axiom only claim 2 Litres.
My main bicycle these days is a folder (Xootr Swift) which has a long length of seat pillar showing. Other bags tend to look a bit out of place behind the saddle with so much fresh air between them and the rear 20" wheel. I also use the bike for day rides of between 15 and 35 miles so do not need to carry much.
So, what do you get for your £27.99?
The bag is quite narrow and sits nicely behind the seat pillar - held in place by Velcro type straps. Two sturdy straps hold it to the seat pillar and two further straps wrap around the seat rails. Sadly the seat rail straps are quite thin and the Velcro material does not cover the full length of the strap material. If you pull a lot of strap through to keep the bag tucked under the seat most of the Velcro overshoots its partner leaving only a few mm of contact between the two surfaces. If you ease back on the straps to make more contact the bag hangs away from the saddle. Fortunately, the other two pillar straps are pretty strong and hold the bag in place.
The side pockets are narrow and the zips to open them do not run the full length of the pockets. This makes getting stuff in and out of them a bit of a task. I made the mistake of putting my rain jacket into the main compartment first and the bulge it created meant I couldn't get anything into the pockets! I had to remove the jacket, fill the pockets, then replace said jacket. I have two inner-tubes in one side pocket and tyre levers, multi-tool, and puncture kit in the other. There is no room for anything else. My rain jacket is medium weight model that I can roll into a size just a bit larger than a soft drinks can. It just about fills the main compartment. If you have a featherweight jacket that scrunches into a pack smaller that a tennis ball you won't have any trouble popping it into the bag along with your lunch. My lunch is up front in a small Rixen Kaul 'bar bag positioned just above the front wheel. If I put the bottle shown in the pictures into the bag's main compartment there would be very little room for anything else.
The bag has a mesh pocket at the back to hold a water bottle. The bottle in the picture is a 750mm one and has a "waist" which matches the elastic around the top of the mesh pocket. Your standard 500mm bottle will be held in there OK but I also have a taller and slimmer Sigi aluminium bottle which does not feel secure in there at all. I've never been a fan of having bottles behind the saddle in case they bounce out or get covered with road grime thrown up by the rear wheel. But the bike has a good mudguard and the bottle pocket seems strong. There is a tag to hang a rear light from but, with a bottle in place, you cannot use it!
So, is it a good buy? Well, it depends on what sort of riding you do. It is designed for bikes with a lot of seat pillar showing like folders and super compact road frames. It is meant to carry essentials so will be no good on an extended tour. Axiom call it their GranFondo and aim it at Sportive riders. Not being a Sportiver myself (spell check says there is no such word as Sportiver!) I cannot confirm whether it is suitable for that form of riding. It suits me and my folder. If I can just get a featherlight rain jacket I could put my jam butties in there too!