I'm heading to Jura next week, my onward plans are not yet set. I've reserved a ferry from mainland to Jura and I'd like to reserve more ferries. In some parts of the world I've traveled, the ferries get canceled fairly often, even for mildly bad weather. Given that the weather is not lovely in Scotland, do ferries often get delayed/canceled?
In general: usually the ferries are very reliable, but cancellations do sometimes happen.
I've only once had to deal with a cancellation in the last couple of years of holidaying around the islands, and that was warned a couple of days in advance. CalMac, who run the majority of ferry services, report >97% of services run as predicted, with ~2% cancellation due to weather and ~0.4% due to technical issues. Note that these are year-round figures, and services are probably less reliable in winter than in summer.
Bad weather is often somewhat predictable, so you may know a day or so in advance if there's potential problems - you can call them and ask if you're concerned. They will also try and contact you if you're booked on a service and it's cancelled or rescheduled.
Reservations are a very good idea if you have a car, especially at busy seasons (like now) and for smaller ferries (like Islay-Jura), or for the longer routes that only sail once or twice a day. I would not personally plan a car journey without them.
To build on Andrew's answer, CalMac also have service status text updates (for free) and an app which serves the same purpose. I would recommend using both to cover all bases as the phone service coverage on the Hebrides is unreliable at best.
It may also be worth noting that there have been some very inconvenient delays to the addition of new ferries to the CalMac fleet which caused multiple cancellations for ferries to the Outer Hebrides. However, this was due to choppy weather preventing the smaller vessels that are currently in use from sailing the longer journeys so hasn't been a problem since around spring time. As an islander, I would say don't be deterred by our grumblings about unnecessary cancellations, they are actually all necessary and really incredibly rare.
Another thing to note is that if there have been multiple cancellations and people have been left stranded, CalMac will do 'emergency' sailings that are off the timetable to help ensure everyone gets to where they need to be. I should emphasize that I've never known this to happen outside the winter months.