I'm planning on arriving in Sicily on the 10th of August and leaving on the 15th. I'd like to hit Palermo, Segesta, Selinunte, Agrigento, and Syracuse, plus Marsala and Taormina, if I can manage it. I had planned on coming by train to Palermo, then heading generally south and east. But train service seems so horrendously slow, I'm wondering if it would be better to fly into Palermo and rent a car?

sicily trains are slow but they go everywhere you want to go and the island is small so you'll only spend a few hours each day on a train.
Palermo-Segesta - about an hour by rail but you do have to walk about a mile from the desolate deserted Segesta station - maybe better to take a bus from Palermo right to the stunning archaelogical site and its stunning Doric temple extant from 430 BC - extremely well preserved and in a wild natural site.
Palermo-Agrigento two hours on train
Agrigento-Taormina via Catania about three hours; day trip Taormina-Syracuse by train - about 1.5 hrs each way. Marsala also has train service not sure about Selinute. I suppose car is more convenient but more expensive as you can buy a Sicilian railpass locally i believe for a really cheap price. Heed warnings on parking cars with luggage in them in towns like Palermo.
These are old-fashioned trains - slow but sweet - and you can lean out the windows!

You have five days and seven destinations. This is not reasonable or realistic. Palermo alone is worth two to three days. Segesta and Selinunte combined might be possible in a one-day bus tour. Agrigento by train is another day, Siracusa another (the fastest way is probably by bus from Palermo in 3 1/4 hours), and Taormina yet another. Marsala, as I recall, is at the western end of the island.

Sicily is small?

Train travel on Sicily is wonderful if you have the time. But to visit all that you want to see in five days, rent a car.

Each to his own style but i did this very itinerary by rail, not including Marsala, which is not on most tourists scopes, in 5 days plus taking the Circumetna railway around Etna and saw all that i wanted to - but granted i may be a more hyper traveler than others. but it's possible given your travel style. To do a similar car trip in five days may be just as 'hectic'.

I think I have a workable schedule.

10th: Arrive Palermo in the morning from mainland. Night in Palermo.

11th: To Segesta, then to Selinunte. Night in Marinella di Selinunte.

12th: To Agrigento. Night in Agrigento.

13th. To Syracuse. Night in Syracuse.

14th. To Taormina. Night train out.

Yes, it's no more than a day anywhere, but that's the cost. I can always go back.

The only thing I'm really not sure about is the Selinunte-Castelvetrano connection. My guidebooks say bus service between them is in frequent. It's only 14 km though, which is frustrating. Perhaps the hotels in Marinella have a car service?

As you may know from my trip report, we just spent 15 days doing that same itinerary. I really think you are kidding yourself. It would be tough to do that with a car--almost impossible using public transport. Given your time I would see Palermo and Taormina.

bob:

Thanks, but I think you have different travel interests than I do. If you look at my itinerary, you'll see where my specific interest on this trip lies. Palermo and Taormina are in fact rather toward the bottom of my list. On this trip, I'm not especially interested it hanging around sipping coffee all day. On other occassions, I would be, but right now I'm just worried about logistical feasibility. I'll have many more opportunities to travel in the future.