With all respect to the parties involved the French deep group (Comnex -sp) had go through some major hurdles to get Hydrogen based gases to play safe. To my understanding it required the bell to be at 100ft before they would even consider using the stuff. They had fire and nasty issues above 100ft. No thanks this is not for me.
The Divex SLS Mk IV test was done by the US Navy 750ft wet dives done in a chamber monitored by divers. All the information is listed on the website.
Description of the activity-
The 17-day (including decompression time) saturation DIVE operation, included 8 divers in wet dives, in 34 *f water, down to depths of 22 ata (725 ft). At the max dive depth, a heliox gas mixture containing about 2% Oxygen was used to maintain PPO2 values of about 0.4.
If anyone wants to refer to this please be sure to credit to Mr. Dan Dunfree. Otherwise it would have never been made public. Also keep an eye out for the documentary.
Web-
http://www.therebreathersite.nl/SemiClosed Rebreathers/UK/divex_sls_mk_iv.htm
If divers are interested in gas and gas usage then rebreathers are great. While having major training requirements and other drawback they are a total blast. I put in a 2-1/2 hour dive with a ppo2 setting of 1.2 and felt incredible when we exited the pool from our introduction dive. Better than breathing nitrox~!
Example- a friend dove to 250ft on his rebreather and did all the usual stops- he used a total of 25 Cu ft of gas (both oxygen and Diluent and I am rounding up.)
So if gas, decompression limitations, expense (or cost) of gas, then there are other options. However, you have to carry the same amount of Open circuit gas for bailout if the rebreather crashes and you have to get off the loop. Trimix fills are in the area of $100 for a set of doubles and double that cost in mexico. Good luck find helium in the pacific basin. However, the cost to fill two 20Cu ft tanks is about $20.00 each. So you gas bill drops signifcally. Bailout costs are constant unless you need to use it.
To date there are a few people doing 500 ft dives on rebreathers and they seem to be working out. However, it needs to be noted that 500ft dives require a team of people, extensive bailout, and safety divers. This is not an easy process done by a single diver.
The ultimate gas seems to be getting nitrogen out of the loop. Many deep divers are using mixtures that require specific knowledge and training to really talk about and to understand issues like HPNS, the shakes, deep water black out, Co2 build up, and other issues. Also we have seen divers including helium in the mix for decompression to stop heavy slamming of nitrogen when they are doing decompression. Having helium content or breathing the bottom gas up seems to be one of the newer trends.