It's sometimes really hard to get an answer on the AD16 (non X)
I bought one because lets face it, they are insane value for money. I also have a Rosetta 800 and sonically they are similar to me. Just great, no complaints. Love them both.
I've not compared them to the X but it felt like a decent step up from the AD8000 aside from the lack of metering.
The only thing to really look out for is the lack of expansion ports. If you're good with AES, optical or TDIF you'll be fine.
If you see a good deal on one I'd go for it!
Don't forget it's a high end converter... even if a new end model is out which may sound slightly better I'm not convinced its of an order of magnitude that would bother me! 16 channels of apogee A/D for £900 is what I snagged and I wouldn't look back!
I had an AD-16 that I sold when I realized what I really needed was an 8 X 8 for surround mixing, so I bought a Rosetta 800. I was told directly by Apogee that the converters in the Rosetta and the AD-16X are upgraded from the AD-16, but I still work in the studio to whom I sold my AD-16 and I can tell you it is definitely a fine converter. The Rosetta has more features and seems to have a bit more headroom, but sonically I don't hear a pronounced difference.