I liked my D26 Whale (1100 lumens). I've recommended it to others and they really like it. When the D26 1600 came out, I got one and was sadly disappointed.
The light I purchased would run for 3 minutes then tell me the battery was dead. I'd take the battery out and check it only to find the battery was fine. I could even put the same battery in the D26 Whale and it would run for hours. I talked to a woman at XTAR but just got a lot of run around and occasionally ignored.
It wasn't until I started talking to @stuartv on here that I found out he was also having problems and THEN someone from XTAR actually tried to help me. At one point they shipped me a sleeve so I could not use a 26650 battery in the light. Only the 18650 battery would fit. This solved the 3 minute problem but now the lumens and burn time were greatly reduced. After weeks of dealing with someone on here XTAR finally agreed to ship me a new light.
The new light wasn't much brighter than the original D26 Whale. So I just use my D26 Whale and a Hollis dive light as a backup. The D26 1600 is in a storage box somewhere.
Bottom line, I'm in no rush to buy a D26 2500 and still won't recommend a D26 1600 to anyone.
There were issues with the 1st release, that's been fixed. However the locking mechanism on the 1600 is still crap...so I still recommend the 1100 any day over the 1600.
To me the 1100 is the perfect light for anyone for diving. It can start as your primary and eventually move to a backup. It's inexpensive, can take 2 different types of batteries, and the output has 4 adjustable modes. It's awesome.