As a few posts back, have you noticed improvements in loading times and do they just stop there? Is online gaming sharper or not noticeable..
Id happily upgrade if there is an actual improvement in performance, online mainly.
http://www.eurogamer.net/articles/digitalfoundry-2014-how-to-cheaply-upgrade-your-ps4-to-2tb
That article shows the speed improvements with some common HDDs. The Samsung one I've got shows minor improvements over the standard one. There's a Hitachi 7200RPM one which is better in terms of speed, but it's negligible, and it's also £50 for 1TB, so more expensive per gig - I thought about going with the 1TB, but didn't want to be filling up another drive any time soon, seeing as games will only continue to get bigger, so went for the 2TB 5200RPM option. You'll see good speed increases with SSD, but the Crucial one recommended is £90 for 250GB I think, so very expensive, and I'm not sure you can get large SSD drives yet, and if you can 1TB+ will probably set you back more than the value of the console.
It's a shame you can't have a small SSD which has the most recent game(s) in it's memory and the boot stuff and then another drive that can store all your extra stuff on and then it puts your recent stuff on the SSD for quicker access. Would've cost more upfront though and more space in the box so not really a good business decision from Sony.
With Xbox One, I know you can have external HDDs, does this mean you could put a small SSD in the console and then store stuff on an ex-HDD?
With regards to online,
@Goodj is right, although you might get better rendering etc with a faster drive, I'm not sure what comes from the internet and what the game itself is rendering. Then again, I've never really noticed rendering issues with the standard drive, and as the article I posted above states, they had to test on a frame by frame basis to find minute differences.
Also, the bonus of getting that Seagate drive is the HDD that comes out of your PS4 can be put into the Seagate caddy and then you have yourself a fully working 500MB ex-HDD should you need it.