One the plus side the views of the mountains are amazing. The Matterhorn was hidden in cloud, but the view of the Breithorn were great.
On the minus side. It is very expensive (even with a 50% off Swiss Pass). The viewing platform was closed on the day we went. The ice palace+ice slide is poor. Don't waste your money. Also you have to go through about 5 turnstiles up and the same amount down, which seems quite unecessary.
Tips. It takes an hour to get up and another hour to get down, so go before lunch if at all possible. Take warm clothes and sun glasses.
The cable car sways quite a lot - so give it a miss if you have vertigo!