I always use them - they don't get too hot and neither of my older girls minded wearing them as they look pretty cool nowadays. The little ones only wore these last year in Malta in August and were fine, we got the matching legionnaire-style hats that they could keep on in the pool too.
When DD1 was little they were a rarity and cost nearly £30 (she's 16 now) I bought mine for the younger DDs in M&S last year for around £15, the supermarkets sell them too.
I would advise separate top and bottom rather than an all-in-one for easier loo trips! (Same reason I put the girls in bikinis if they're not wearing the sunsuits)
I got long sleeved and legged ones from tesco for £9 this year. We took them to Tunusia and the girls lived in them. Great as only needed to suncream face, hands and feet. Plus they are really bright (turquise and pink, ick) so I could spot them a mile off!