paraiso had some other bad info in that post. The express bus from Monteverde to downtown SJ is scheduled to take 4.5 hours which is exactly the same as the schedule transit time for Interbus. Yes, it makes a few stops along the way (most of them after the airport), but so does Interbus at the beginning and end of the trip as you stop to pick-up and drop-off the other passengers at their various hotels. Of course, getting door to door service is a nice perk but you pay for it at $29/pp versus <$5 on the public bus. Also, it should have been pointed out that you can take about 30 minutes off of the estimate if you're only going to the airport rather than all the way to downtown SJ.
Also, I didn't understand the comment about the airport rental agencies and having to take taxis. If that was at all an issue you could simply have stayed at a hotel closer to the airport or at least outside of downtown. MAny people prefer to do that anyway. Besides, even if you couldn't have found a quote for $110 in Fortuna, $170 would have been excessive since you could have just kept the car paying a little extra for a partial day, but also offsetting any taxi fees you might have otherwise needed to pay for and giving you the flexibility to maybe do something else interesting with it for your last morning, and still coming out cheaper than $170 when you finally returned the car before your flight.
Finally, the public buses are just as comfortable and virtually as quick as taking the shuttles. Basically sitting in any vehicle for several hours while driving on a bumpy road is going to have a degree of discomfort no matter how you go about it. What the public buses really lack compared to the shuttles is the convenience of door to door pick-up and drop-off versus having to find and wait at some smelly bus station or bus stop. So thats what you have to weigh against the cost differences when you decide between the two. And whether you drive yourself or have someone else do it, you'll still be subject to traffic delays along the road. Yes, drivng yourself or hiring a private driver will be a little quicker than any form of public transport where you are forced to make stops for others, but you're only talking maybe 30 to 60 minutes extra max for this trip. Again comfort factors are not appreciably different while you're on the road, with the exception that with yourself or your own driver you can stop anywhere you want along the way to work out the kinks.
That gets to the one REAL important difference between these various methods and that is not comfort or speed vs. cost but rather convenience vs. cost. With either the bus or shuttle you're subject to their schedules. You're more limited in when you can leave. There are only 2 scheduled departures from MV each day by either bus or shuttle. And you're much more limited in when and where you can stop along the way. With private transport you can leave anytime you want. With the public transport you go their route and you stop only where others need to get on or off. Going by way of La Paz is completely out. With private transport, you can take any detours you want and stop to admire views, rest, eat, etc. anywhere your heart desires. For many people that is well worth the hefty premium you pay for a car or driver.