I've got the 7350. Impressive printouts, but the only downfall is the photo printing. You can't do boarderless prints on it.
Ink efficiency all depends on the quality of the printouts that you set. If you print a lot of photos or pictures, the colour cartridge will finish fairly quickly.
I think it's always best if you change the default settings to 'economy' mode when printing all the everyday stuff like school work, and 'normal' quality when it comes to printing important documents. This would save a bit of ink for you.
Edit: One thing I forgot to mention is that with the HP 7000 series printers, there seems to be two settings that can change the quality of the prints. One is the 'economy; normal; best' modes and then there's this other one which determines how 'fast' the ink on the printouts should 'dry' - 'short' or 'long'?
This basically means that for faster drying time there will be less ink used when priniting.