There is no doubt that Villa d'Este is a beautiful site, a fine house with gardens sloping down a hillside overlooking the Italian countryside. The town of Tivoli itself, surrounding the villa, is rather spectacularly set in the Sabine Hills east of Rome, overlooking the falls of the Aniene River. It is quite easy to see why this area was favored as a retreat by church and state in Rome -- it feels a world away from the eternal city.
I visited Villa d'Este in November last year. Perhaps autumn is an off season, but I had to adjust my hopes for this site when I arrived at the ticket office and learned that none of the fountains would be running, as they were under repair. For me, this was a bit like my first visit to Versailles, a weekday with no grand fountains on display. Except that the villa is in no way comparable to the richly ornate Palace of Versailles. The fountains are the main draw to the Villa d'Este, and without them, I was left wanting more.
That said, the gardens on a fall day were beautiful to walk through, and I could imagine what the fountains would look like were they operating in their cascades and grottoes. The gardens also weren't terribly crowded in November (perhaps because visitors knew in advance the fountains were not working?), so I never felt crowded wandering the grounds. Still, just like the Palace of Versailles, I know I'll have to return one day to experience the full beauty of this site.
Logistics: The Villa d'Este is an easy walk through town from the Tivoli train station; Tivoli can also be reached from Rome by bus or via private transportation.