Not used it mate but have been there many times. There are 2 the main one by the RNLI lifeboat station which is very steep! Looks the the people who have thier boats there use a tractor to get it in. Im not sure if there is anywhere to tie up once in the water
There is also a second one tucked away opp the activity center now this one has a much shallower gradient but is very narrow but doable in a car. This one also has a pontoon at the bottom that you can tie up to but is subject to tide
These both come at as cost pay at machine i usually Bucklers hard but my mooring is there other wise its a couple miles out to the solent but a nice plod down the river
calshot..i have used the end slip (the steep one ) a few times. 17.6ft boat and had a ford galaxy at the time. no problems. at the time it was £11 to launch and for the parking and a good flush and wash down after wards. the good thing about it is your on the fishing nearly straight away.
there is also a good slip(tarmack) in southampton next to the itchen bridge.crosshouse.. free launch and trailor park but have to pay for car park. and your mostly out of the flow so quite easy to get back on the trailor. but its a bit of a run out to calshot from here.
Don't mind paying to park or launch I wanted to use Lymington slip but it doesn't reach all tides
Calshot was the next best bet for full tidal range. Also has the added benefit of wash down but not a pontoon to tie up too unless I motor down to the other shallow slip and tie up there to park the car