I use one of the Pure Freedom trolley systems, one of the older heavier models rather than the ones they sell now.   They're generally pretty good and as in my case I only use it for upstairs windows, I only need to bring it inside to charge once a week.  
It does give me the advantage of being able to do back windows of properties where the only option to get around the back is through the house.  It is a hassle though lugging the 3 separate parts through, (as in pole, tank and trolley).  As mentioned, if using a 25 litre tank or larger, they can be very heavy when full when lifting in and out of the vehicle.  I generally use smaller tanks now to get around this.