Does the Caulfield to Clayton shuttle bus get full easily ? And if any of you have used it , approx how long does it actually take ( website says 15-25 mins)? Cause I'm planning to use public transport till I get to Caulfield and then use the shuttle bus. Should I just use public transport completely ? Just trying to think ahead. Thank you 
Using PT to the shuttle bus and then taking the bus to Clayton is a great idea, I do it myself. I take the train to Caulfield and then to the bus. The shuttle normally takes 13-15 minutes during the day at off-peak times. And 20-30 minutes during peak times, with an average of 24-25 minutes during peak times (5-6pm). Keep in mind if you go to Clayton in the morning and from Clayton in the afternoon you are generally going against peak-hour traffic so it isn't so bad and the shuttle bus uses the express bus lanes.
To take the train from Caulfield-Clayton you need to get to Huntingdale station (17-18 minutes from Caulfield and then take the 601 bus 8-10 minutes to the Clayton bus loop, so the shuttle is definitely quicker.
The shuttle also has free wifi

The shuttle is normally 30-40% full during the day and gets pretty packed at peak times in the afternoon 4:30 till 6ish. Get there 5-10 minutes before the bus is scheduled and you will be guaranteed a seat though, it's only the stragglers that have to stand or wait for the next bus. When the bus is full it can be a bit uncomfortable but not that bad.