The short answer
Yes. Split is worth two to five days, and it repays them better than almost anywhere else on the Adriatic, because it is three things at once: a Roman emperor's palace that people still live, drink and shop inside; a swimming city with beaches a walk from the old town; and the ferry port for Hvar, Brač, Vis and Šolta. Come in June or September and it is hard to be disappointed. Come for a quiet beach week in mid-August and you may well be.
What makes it worth the trip
Diocletian's Palace is not a fenced ruin. It is the old town — free to walk through at any hour, with a bakery, wine bars and flats inside walls built around AD 305.
You can swim before breakfast. Bačvice is a few minutes' walk from the palace, and Kašjuni and Bene are on the Marjan side, by bus 12 or on foot.
Marjan, a pine-covered hill at the end of the Riva, gives you a forest walk and a city view fifteen minutes from the harbour.
It is a base. Hvar, Brač, Trogir, Krka and Omiš are all comfortable days out, so a week here never repeats itself.
Start with Diocletian's Palace, and walk it early or late.
By season
June and September: the best of it. Warm sea, long days, every boat running, and a table without booking three days ahead.
July and August: hot, crowded and at peak prices, especially mid-August when Croatia itself is on holiday. Worth it if you want the full summer — festivals, late nights, every island open — and can plan around the midday crowds in the palace.
April, May and October: the city at its most pleasant to walk, cheaper beds, a sea that is cold in April and swimmable from late May into October. Some island boats run less often.
November to March: a quiet, local city of 10–15 °C with cheap rooms and an empty palace. Worth it for a city break, not for a beach holiday.
Month by month: the best time to visit Split.
By traveller
Couples: yes — sunset on Marjan, dinner in Varoš, a day on Hvar.
Families: yes, if you stay near Firule or Žnjan rather than inside the palace walls, where the bars run late.
Groups and nightlife: yes in summer, with clubs and beach bars busy until three. Out of season the nightlife is quiet.
Beach-first holidays: only partly. Split's beaches are good city beaches, mostly pebble and busy; for long empty bays, base yourself on an island.
Culture travellers: very much — the palace, the cathedral, the Meštrović Gallery, and Salona and Klis just inland.
When it is not worth it
If you have a single day and it falls between eleven and three in July, when the cruise groups fill the Peristyle, you will see Split at its worst. If you want sandy, quiet beaches, Split is the wrong city. If you want a holiday where you never queue, avoid the first three weeks of August. And if you are choosing between one night in Split and a straight run to an island, the island may serve you better — Split needs at least two days to make sense.
How long to stay: how many days you need in Split.
If you only have a day, follow Split in one day.
Weighing the alternative? Split or Dubrovnik.
The money question: is Split expensive?
For the swimming: the best beaches in Split.
Things to do in Split
Split in two days