I've had plenty of opportunity to use Siege frigates, to great effect.
The effectiveness of the SF, in my opinion, entirely depends on the enemy. Every fortress, every defense has a weak point somewhere, and SFs are the best option for exploiting that.
I think most people don't like SFs because their expectations of them are too high ... most people want to have a brutal planet-slaying, but I think it's also acceptable to simply use a well-timed strike on an undefended planet to kill the population. You can sometimes accomplish this goal before the enemy has a chance to respond, depending on the dynamics of the map and where the enemy fleet is. Murdering a planet's population destroys its income, for an appreciable amount of time. Depending on the success of your attacks, and the targets, you can take out anywhere from 3 - 10 credits-per-second of planetary income. Even better for TEC if you have the radioactive fallout ability, making repopulation come to a crawl.
To make a comparison to say, Starcraft, this would be like killing all of a player's SCVs, but not the Command Center. It doesn't necessarily win the game, but it certainly gives you a temporary leg-up.
My favorite scenario is when I find a cluster of planets that the enemy has neglected to defend well. You send in 6 SFs and murder a planet. Unless the main fleet is literally next door, they'll try to take out your SFs with a small force of a few LRM frigates or whatnot. When they do, you scatter the group - send the 6 SFs in groups of 2 in 3 different directions, if you can, and hit 3 more undefended planets. Keep going back and forth like this for as long as you can. The economic damage you can achieve if you pull it off well makes the frigates pay for themselves, if you ask me.
Not only does it hurt the income, but it can usually cause the enemy to get very paranoid/annoyed about SF strikes. I find it funny to come back to a planet I've bombed 2-3 times and literally see it ringed with about 10 cannons. What's even funnier is when I use the ALT key to check the ranges on said cannons, and see that there is one little spot he missed. Hangars are definitely the better solution, though can be easily countered if the SFs are accompanied by flak frigates. The best counter, I think, is to couple such defenses with a single frigate factory, so that you can spam a few support units to counter those sorts of strikes when you need to.