I've used Fusion Drive equipped iMacs more than a year old and they're still very fast to boot up and launch applications, even with the mechanical drive more than 50% full of other data. If you aren't into external drives and can't be bothered to manually organize your data, it's the way to go. It hasn't got the speed of a pure SSD setup, but a lot of users won't know the difference in day to day use.
__________________ ACMT MacBook Pro (15-inch, Mid 2012) 2.3 GHz Core i7, 16GB, 256GB M4 SSD iPhone 8 - 64GB S/G • Sound System Audio Engine A2 • Display UltraSharp U2412M 24" Custom Built Gaming PC Kaby Lake Edition | 16GB DDR4 | GTX 1070 8G
I just ordered the 2.5" drive adapter from OWC as I am going to replace the existing 500GB drive with an SSD. I've been getting some drive errors using Drive Genius for the last while anyway so I may as well replace it. I could have replaced the DVD but the macbook air doesn't have one and neither does my kids' Asus Transformer so we are DVD-less for the odd DVD disk that shows up with school photos/videos etc.