I use the RainX Glass Cleaner - Basically glass cleaner for when you have finished cleaning your car, with the added benefit of the RainX effect! It as about £2.50 from Halfrawds! Will easily last a month, but I tend to use it after every car wash.
Works brilliantly, I do a lot of motorway miles, around 35K a year, and as mentioned anything over around 45MPH you do not need to use your wipers, the rain just glides up the screen.
I have found that the it does smear, but the smearing I get with it on the screen is much less distracting than the smearing I get without out it on the screen. If you have got new wipers then the smearing goes away virtually. Mine are in sh1t condition.
All in all go for it, well worth it. I think the value for money on the Glass Cleaning version is good, as I am not buying both glass cleaner and RainX, just buying one product which is the same price as normal glass cleaner.
There was a thread on here about another product - I forget the name - that was meant to be much better than RainX, lasted 18 months, and did not smear. Might be in the detailing section.