It turns out that I was indeed scammed.  There is a very good fake of bitmixer.io - running under the URL bitmix.io.  It has a valid SSL certificate and with the similar domain name I missed it when I was doing my usual checks.  The key is to always verify the letter of guarantee before sending your coins.

Again - beware of SCAM SITE BITMIX.IO!

It is not affiliated with the legitimate site bitmixer.io.

I initiated a transaction with Bitmixer.IO today and copy and pasted my destination address in to the page.  I then copy and pasted the input address that was generated to my wallet application and sent 2.4 BTC.  I downloaded the letter of guarantee but did not read it before sending my payment.

When I didn't receive my coins I checked the letter of guarantee and the output address does not match my intended output address which should be:  1EBNnrDmgmnpHSNcP7BfqB7YEYoc2E7kS

I also then noticed that the letter of guarantee also has the wrong date shown in it.  I then checked the time on the file on my computer and it was indeed downloaded today.  It appears that my transaction has somehow been botched.

Below is the letter of guarantee.  If you check the input address in the blockchain you can see that the 2.4 BTC was sent.  If you check the output address listed in the letter it is still empty.  Additionally I have not received the coins at my intended output address.  (You will see an unrelated transaction depositing 2.4 BTC in to my intended address as I needed to complete the transaction when bitmixer.io failed.  But obviously those are not the coins from bitmixer.io as the fee amount would have been deducted from the final output amount.)

I have tried to use the support form on bitmixer.io to report this but the form doesn't work.  I'm hoping someone will see it here and be able to assist me.

-----START SIGNING BITCOIN ADDRESS-----
1BitmixerEiyyp3eTLaCpgBbhYERs48qza
-----END SIGNING BITCOIN ADDRESS-----

-----START LETTER OF GUARANTEE-----
We hereby confirm that BITMIXER.IO has generated the address 1DA4hzMHLnaw6hYyZD9PdtFQ4LsuPMFkaU in order to transfer incoming amount (minus fee) to the following addresses:

100% to 1N6T1QQG6tGnAMTFHwgb84ASBKoroM22bN

This service will be only available for all bitcoins received from April 14, 2015, 14:25 to April 15, 2015, 2:25 GMT with minimum amount of 0.01 BTC. Our fee is 0.7132% + 0.0005 BTC for every forward address.
This letter is digitally signed by our main account: 1BitmixerEiyyp3eTLaCpgBbhYERs48qza. Stay protected and thank you for your services.
-----END LETTER OF GUARANTEE-----

-----START DIGITAL SIGNATURE-----
GzrUf/JUe9IiNReoowZtk0g4whVZB/d1mQ7ULQsHP9RruSEwNH/A/kHQhXK458AhViGp3JVw+2WByS+RN5+MbQA=
-----END DIGITAL SIGNATURE-----

I've been trading bitcoins for a while but haven't used them for any purchases.  Recently I've considered using them to buy hardware from various individuals on the internet but of course I want an escrow of some sort to avoid the scam artists.  Does anyone have suggestions on which escrow site is best?  It seems that btcrow.com is the most popular and has a relatively low fee at 1.5%, but I've also seen a number of others out there.