However most of the time a logic analyser will not be being used at full speed, so large RAM at lower speeds would be very much more useful than a fixed 2K.
Only if you use ancient stuff or low-end 8-bit micros. The top speed for a streaming USB bulk transfer logic analyzer is 24MS/s. If you want any timing accuracy at all, you're limited to <= 5MHz or so. Tons of interfaces are faster than that, even in the low-end embedded world. Both types have their uses, but I wouldn't call the slow 'unlimited' depth type very much more useful, although price/performance might be better.
The problem with not having much RAM attatched to the FPGA is that the timing on USB, even USB2HS is unpredictable, and 2K is nowhere near enough to ensure no buffer overrun. Adding some cheap RAM would make a USB analyser way more useful.
USB is nowhere near fast enough for 500MS/s, these types of logic analyzers are usually designed to just fill the internal memory, stop acquisition and transfer. So buffer overruns are by design.
A 4Mx16 133MHz SDRAM is about $1, once you account for address setup overheads, you can get maybe 100MHz sustained transfer. A single 16 bit wide SDRAM could therefore give you 200MHz sampling of an 8-bit bus.
You'd probably need a more expensive FPGA to handle SDRAM. You might also need more than the 2k buffer to counter the latency. For 32-bit wide access, you would need tons of chips. And I'm skeptical about the $1 figure if you have to buy it in commercial quantities (can't just get them from a dusty shelf in the local computer store).
I don't really see the appeal of this over the Intronix Logicport, which is well known and has proven itself. I would take a hard look at the software, because most software from low-budget Chinese companies is pretty bad. The Zeroplus software was absolute crap last time I looked at it. I also wonder if it has the same features (both timing and state analyzer mode, adjustable logic thresholds). Another interesting option is the
Open bench logic sniffer, it's only $50 or so and the software is maturing quite nicely. It doesn't do 500MHz (200MS/s for 16 channels), but has more RAM. The software is maturing quite nicely.