For the last few months I've been using a glidetrack sd which you can purchase from the website www.glidetrack.com. I bought the SD range compact ($300 including shipping) and I think it's the perfect size for HDSLR shooting. Although the company is in the UK, I was able to talk quite easily to the one man company (Alastair Brown) who gave me tips. I was surprised to find the glidetrack got to my home in brooklyn in less than a week from the time I ordered it. Better than I can say of some companies in the US.
TIPS: I put a quick release plate between my tripods legs and head. then attached the plate to the bottom of the glidetrack. this allows you to quickly snap on the glidetrack and snap off.
also you can use this as a small jib, by putting the glidetrack on the your sticks then attaching another head onto the top of the glidetrack. very handy.