The bug and error I encountered were quickly resolved.

No account needed
Whenever possible, I prefer to exchange without making an account, so I didn't Register. I like this, it's what makes instant exchanges stand out from the classic ones.

Test transaction
I used the onion site for my test. The exchange rate is about the same as for instance FixedFloat. I'm not sure if the withdrawal fee is already included in the estimate.
I think the "Calculate approximate cost" button should work in both directions. I can't get an exact amount, because the exchange rate changes all the time. I can choose how much I deposit, so it would make sense to base the estimate on that amount and tell me how much I would receive approximately.

I deposited 0.00025 BTC. It took some trial and error to find how much Monero (0.029173 XMR) I'll get for this:

After a while, I received 0.029058 XMR.

Slow
Most of the time, I use instant exchanges for small amounts, when I quickly need something. Waiting for 5 confirmations makes a competitor more attractive, even if their rates are slightly worse. I wasn't very lucky, when I checked after almost an hour, my Bitcoin deposit had only 3 confirmations.
Can you lower the number of confirmations for small transactions to 1 (and maybe require more above a certain value)? Even though many large exchanges (the ones where you need to create an account) also ask 4-6 confirmations, most instant exchanges send funds much faster. After all, it's in the name: instant exchange.

Making a typo
When I accidentally used a comma instead of dot in the estimation field, I got this:

Error handing should be improved (by notifying the user on the same page).

USP
I can imagine there's a demand for a Bitcoin <> Monero exchange without KYC on a .onion domain. If you don't ever ask for KYC, you should definitely advertise that on your site.

Design
Unlike most people, I don't mind the simple design of the site. But there should at least be some information on how to contact anyone if something goes wrong.

Fryx
What does the name stand for?

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When using instant exchangers, I usually use BestChange to check a few of them for their rates. Because I don't trust many of them, I make only small transactions (and if needed, I do that more than once). I often choose one that isn't the cheapest, but gives me the best impression (mainly no KYC). So from my perspective, reliability is most important, small changes in rates come second.