I normally avoid discussions about specific strobes, because I think light is the most crucial thing in photography (I know how valuable having exactly the light I want has been to my career). And the right quality of light is far more valuable than the quantity. However there are no measurable specs for quality - so photographers argue about the measurable specs. However, I know what light I want and need and that informs my strobe choices.
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I shot a pair of Inon Z330 strobes on a trip to Cayman in January. That trip persuaded me to get the Retras (although I haven't got them yet). But I would add that both strobes are a significant step on from the previous generation strobes like the Z240 and D2.
On that trip I used the Z330s most of the time (as I had used the Retras before and had a good knowledge of their performance) - which is how I assessed them, but did one dive with one of each strobe to compare them in a single shot to share. This shot is not what I based my own conclusions on - but it is consistent with them. This is the comparison shot of the two strobes (that I posted on Facebook at the time) - screen grab from Lightroom:

Quotes from that post"Comparison of coverage and quality of light between single Inon Z330 strobe (left) and single Retra flash (right). This is not a comparison of strobe power - I did other tests for that. The Inon was slightly ahead on strobe power, but that may be because the diffuser is does not spread that light as wide. Both strobes are considerably stronger and have more coverage than an old Z240. Do consider that both strobes have done very well in lighting a large wide angle scene, shot into the sun, with just one strobe, which is very impressive. These were both shot on about 50% power. Taken with Nikon D5 and 8-15mm fisheye @ 15mm, f/11 @ 1/320th, ISO 400."
"The difference between the strobes was immediately obvious testing them together. With diffusers attached, the Inon is about 1/3 stop more powerful than the Retra. I did the tests with diffusers on because you'd always use them that way for wide angle. This difference is almost entirely down to the difference in the diffusers. The Z330 currently feels a bit unfinished and rushed to market. It needs a better diffuser - one that spreads the beam a little better and warms the light (it is currently way too cool - and you can't add gels because of the domed front)."
and
"Strobe manufacturers like to quote Guide Numbers - which is the most useless information there is. This is because the guide number is inversely proportional to how wide the strobe beam is set to be. If you want a high guide number, simply make a narrower beam. Easy.
Unfortunatlely the best light for lighting wide angle scenes in clear water is wide and soft - so that it illuminates the subject naturalistically, without eyecatching burned-out hotspots and harsh shadows.
Some manufacturers do quote angle of coverage - which is good. But again it is not the best statistic. What is more important than how wide the beam is - is how much of a fall off of light is there from the centre to the edge. Two strobes might both have a quoted 120˚ coverage, but in one, after 90˚ there is only 20% of the light remaining. In the other there is 70% remaining. The second example will be easier and more flattering to use."
Quotes from that post"Comparison of coverage and quality of light between single Inon Z330 strobe (left) and single Retra flash (right). This is not a comparison of strobe power - I did other tests for that. The Inon was slightly ahead on strobe power, but that may be because the diffuser is does not spread that light as wide. Both strobes are considerably stronger and have more coverage than an old Z240. Do consider that both strobes have done very well in lighting a large wide angle scene, shot into the sun, with just one strobe, which is very impressive. These were both shot on about 50% power. Taken with Nikon D5 and 8-15mm fisheye @ 15mm, f/11 @ 1/320th, ISO 400."
"The difference between the strobes was immediately obvious testing them together. With diffusers attached, the Inon is about 1/3 stop more powerful than the Retra. I did the tests with diffusers on because you'd always use them that way for wide angle. This difference is almost entirely down to the difference in the diffusers. The Z330 currently feels a bit unfinished and rushed to market. It needs a better diffuser - one that spreads the beam a little better and warms the light (it is currently way too cool - and you can't add gels because of the domed front)."
and
"Strobe manufacturers like to quote Guide Numbers - which is the most useless information there is. This is because the guide number is inversely proportional to how wide the strobe beam is set to be. If you want a high guide number, simply make a narrower beam. Easy.
Unfortunatlely the best light for lighting wide angle scenes in clear water is wide and soft - so that it illuminates the subject naturalistically, without eyecatching burned-out hotspots and harsh shadows.
Some manufacturers do quote angle of coverage - which is good. But again it is not the best statistic. What is more important than how wide the beam is - is how much of a fall off of light is there from the centre to the edge. Two strobes might both have a quoted 120˚ coverage, but in one, after 90˚ there is only 20% of the light remaining. In the other there is 70% remaining. The second example will be easier and more flattering to use."
That said, the deal breaker for me with the Z330 is that I could not use it for inward lighting (p122-123 and p152-153 of my book) - because it does not have a sharp edge to the beam to exploit. So the decision was actually reasonably easy.
Alex