Yep,
Got a very similar problem here too. Normally I switch my MacBook on, Apple logo appears, then login screen.
Since 10.10.3 it takes at least 30-40 seconds before the Apple logo appears. It just sits with a grey screen.
Are you guys using the Trim Enabler app for a non OEM SSD? If so, did you turn it off before updating?
No I'm not using Trim Enabler either.
I can eventually sign in after the Mac has sat on the grey screen for 30-40 seconds.
Try a PRAM reset just for the heck of it.
Try a PRAM reset just for the heck of it.