The castle itself is really quite spectacular but you do need to pay to see it (£10pp or free for national trust members). In truth the best part is walking the causeway at low tide as it's almost magical. You can get a boat across should you miss low tide. Whatever side you're on i recommend finding a spot to watch the tide come in - not only is it fascinating to watch the landscape change so quickly but it's also hilarious watching so many people race to beat the tide!

Missed low tide so had to take the boat both ways but it was a very reasonable 2.00 pounds each way. Short ride but fun when the water is choppy. We made the mistake of parking at the first car park which charged 4.00 and had to walk quite a bit further than the lot down the road a ways that was only 4.50 and much closer. We weren’t sure where to go but followed others. You walk across the sand to the stone outcropping to catch the boat. The Island is very clean with lots of public toilets. Staff were all very patient and helpful. Views were fantastic. Food prices reasonable. Great overall experience.

My husband and I and 2 teenage children visited St Michael's Mount during our holiday in Cornwall in the last week of March. We were staying about an hour's drive away so we combined our visit with a visit to Land's End (30 minutes drive from St Michael's Mount).
We timed our visit so we could walk across the causeway to St Michael's Mount but then had to catch a boat back once the tide had come in.
We all thoroughly enjoyed our visit, as we entered the village we were given a free map of the attraction by a member of staff. The lady who sold us our ticket was lovely and friendly and worked out the cheapest ticket for us (£25 for a family ticket). We were given a detailed leaflet of the rooms etc in the castle and what to look out for for free.
It was great walking up to the top of the Mount as we were able to do so on the original path, which was steep, slippery and wet in places, and once inside the castle it was possible to get up close to the exhibits, things weren't shut away in display cabinets etc, and there were members of staff in most rooms who were very knowledgable about the Mount and its history and very happy to answer any questions.
A lot of places that you visit these days are a bit too 'health and safety' for my liking, but St Michael's Mount allowed visitors access to most areas and the exibits weren't hidden away behind glass screens/in glass cases.
Great value for money, an excellent day out.

Be sure to catch the tide right so you can walk over, don’t worry about coming back there will be fishing boats that drop you back for £2 if the tide has come back and covers the path.

Lovely gardens and views from the mount, plus a lovely cafe with outdoor seating with a great view back to shore.