I looked at these along with the Lee's Ultimate Gravel vac and the Python but ended up ruling them out because I hate things which attach to mixer taps. I instead went for a hose and a pump along the following lines...Not only can I use this for emptying/refilling, I intend on using it to water the garden from the water butt (I have a v. small garden) and I can empty the tank wat directly into the garden.
Here's a brief idea of how it's done...
Equipment
Eheim universal 1250 hobby pump takes 12mm tubing. I initially chose a cheap 400lph pump, but it couldn't cope with the head height - my kitchen has a step down and the difference in height from the sink to the tank was too great for it. The 1250 was a little smaller than what I wanted, but it was a good offer (2nd hand).
6 metres of eheim 12mm tubing - I chose eheim because I knew I could rely on the size of the tubing and most other tubing (non-aquatic) was too vague on sizes for me to rely on it before buying. The branded stuff was obviously dearer, but I chose reliablility in this instance.
A 12mm eheim inttake pipe. You could use a shepherd's crook instead, but the U shape is much tighter on the inttake so it fits snugger over the edge of the tank and is less likely to move.
Some velcro ties to secure the hose when not in use and to secure the inttake to the tap when emptying.
Emptying
Refilling
The pump does have the facility to run out of the water, but the piece needed is missing. I'm ordering a new one (for about £2.50) and when I have it, I hope to make the process a little easier, plus I hope to use this to empty into the water butts in the summer, but also to water the garden from the water butts.
Of course, it's a relatively simple matter to use a hose attachment from the tap before somebody points it out, but as I have a dislike of tap attachments this is a safer method for me!