I don't have a J-series bag, but this insert is great. Easily can fit an SLR body (with a prime attached, such as D90 with 35mm f2, N80 with 50mm 1.4, etc). Can also hold a decent size lens (with room to spare), or a two flashes (SB-600+SB-800, etc) or a lot of other stuff. Use it to divide a larger compartment of a bag and keep stuff from bumping into each other.
It's worth noting that this insert is tapered at the top, so expands at the bottom. it is padded on all sides (includes bottom). It also opens up to fold flat (nice feature for traveling). Padding is not removable like most other Domke inserts (no big deal).
Use this to add padding in any bag you own (not just photo bags). Since it folds flat, it's great for travel as you install it once at location or break it down once at location (depends on your use).
Well made, perfect amount of padding (not too much or too bulky), flexible.