We have tried a few back packs and without doubt the Numatic is streets ahead of the rest in terms of performance (electric lead type).
The downside is its a heavy old lump and no one likes wearing it for any length of time, straps cut in under the arms etc no matter how much they are adjusted. Perfect bit of kit if only used for short cleans or tight areas where there are lots of obstructions.
The battery ones trialed (bear in mind they were to replace Henry Numatics tubs in daily office cleaning environments) never stood the test of time. Battery life was ok to poor and if the cleaner didnt put on charge at end of shift........yep, no vacuum for the next evening!