I have used the Adjustable Grip Hitch many times. I personally like it better than the Tautline Hitch, and the Tarbuck, and the Midshipman's Hitch (all these are sliding/locking loops).

But in my opinion, there are a couple of sliding/locking loops that are better still than the Adjustable. My favorite being the "Locking Loop" (slip it for ease of untying). I have not seen this knot published on the web, but I'm still looking around. It can be found in a book often seen in the discount book section of large retail booksellers (Barnes & Noble, Borders, etc.) That book being "The Directory Of Knots" by John Shaw. The Locking Loop is on page 136. It is simple to tie.

I would recommend picking up this book if you can find it cheap (less than $10). It's a pretty good basic reference. But the king of knot books is "Ashley's Book Of Knots". It's a very good reference, but quite expensive ($50 - $75 depending on where you shop). I got mine at Amazon for around $50 IIRC. Big, hardback, heavy, maybe two inches thick! If you're new to knot tying I would not recommend Ashley's (commonly known as "ABOK") quite yet. You'd probably get lost in it.

Another slider that I think is better than the Adjustable is the "Exploding Sheet Bend". This one can be found on page 145 of Shaw's book I mentioned above. It has been published on the web as well. One such web publication is: http://www.earlham.edu/~peters/writing/explode.htm#4 This knot is a tad harder to tie than the Locking Loop, but not too much. All instruction I've seen on tying this knot shows it slipped by default, including the instructions on that web link above.