Jermaine Curtis (former MLB player) has made several videos using a tee for Smash It Sports. And I have seen multiple videos of Ryan Harvey using one as well, both indoors and out.
While it cannot help with your timing like live pitching can, it does help with perfecting your swing mechanics, hand-to-eye coordination and muscle memory.
Best tees I have seen are (in no particular order). Tanner Tee Pro, Atec T3, G Tee and Champro Brute. All of them have metal tubular stems with a flexible cup to hold the ball. And they are the most expensive. I have the Champro.
If you want to go the cheap route until you are sure, the Easton Deluxe batting tee (I still have one of these too) sells for about $30-35 and is the best of the old style tees, which have one piece rubber stem and cup. If you go this way, I would suggest three simple things:
1. Add a layer or two of duct tape to the cup because the cup will split after repeated hits.
2. Use a 2 foot piece of 3/4" PVC tubing (about $3 from Home Depot) to keep the rubber stem from bending and the ball from falling off.
3. Put some weight on the base to keep it from tipping over. 1 to 3 pounds will usually suffice.
When hitting off the tee, I do not hit into a backstop because it damages the softballs and the fence. Nor do I hit into a net because I want to see the full result of each swing. Not much of a chore for me to shag a bucket of 20 softballs at a time anyway , and it gives me a chance to catch my breath. But you can decide on that for yourself.
Hope this helps and perhaps others who are more knowledgable than me can provide better insight. Good luck and happy hitting.