We frequently rent cars on our trips, including the islands we have visited. St Lucia is the only one we would NOT rent again
The rental car was miniscule and stick shift (which was no big deal). We were heading to a resort (from the cruise ship port) about 35 miles (Anse Chastanet).
It took more than 2 hours to get there, and the last mile or two were the worst roads I have ever seen. You thought the car would not even make it and that you should be walking.
The roads are narrow and windy and you never really get past second gear.
No wonder they FLY people to so many resorts there!
Also, we have only visited one side of the island, but the beaches are few and far between -- lots and lots of high mountain roads and cliffs.
The water is beautiful and clear, and there is a lot of areas that are very lush, and there are a lot of areas there are terribly poor (worse than other islands we've been to).
I know they have some beautiful resorts, so you should have a great time.
However, I would get some more information about where you want to go before renting a car.