This EP was what really hooked me on Sims 3 (pardon the pun)
I had dabbled in the Sims 3 - and enjoyed every moment of it - but I'd never committed to a certain family or location until Island Paradise. For whatever reason, the possibility to explore underwater scenes via SCUBA diving made me really eager to pursue as many opportunities in this EP as possible. There are mermaids to befriend, mysterious islands to unlock and discover, sharks to avoid, and resorts to manage... you can even live on a houseboat and mosey around the network of little islands and atolls that make up the town. Super fun!
This was also the first EP where I found it really easy to get a lot of simoleons quickly, and I definitely think that helped get me immersed in the gameplay. One of the resorts can be "bought" for free and thereafter - using funds your Sim has already accrued - be upgraded. Boom! Before you know it, you may be raking in 3,000-6,000 simoleons a day, and that means exploring other aspects of Sim life - raising a family, trying to unlock islands, working on skills - can be the primary focus.
Hint: it pays to pursue SCUBA diving because it yields great rewards: treasure chests with contents worth thousands of simoleons, as well as diving work opportunities that amount to catching something like 2 squid (worth just over $1000 on their own) and receiving 10,000 simoleons as recompense. Ridiculous. Money like that certainly helps boost the resort ratings by making upgrades affordable!
In any case, after this EP, I was truly hooked. I've combined it with many others, including Seasons, and I get a giggle out of seeing the island paradise blanketed in snow... :)