San Francisco’s Transbay Transit Center began its first day of regular service with shrugs from bus riders and rave reviews for the rooftop park.

“I think it’s super great, cool and fun,” announced Miles Sckolnick, 4, a declaration accompanied by eager nods from his sister Elizabeth and their friend June Tenney, both 7. “Everything is super cool!”

The three charged up a small grassy hill near the center of the 5.4-acre green space and clambered across the roped netting of the lone play structure. They chased the rippling spurts from the 1,300-foot-long fountain designed by Sebastopol artist Ned Kahn, a water feature triggered by movement of buses below.

The fountain was the only link between most first-day visitors and the reason that the lush park exists: It sits atop a $2.16 billion facility intended to improve bus service from the East Bay and, someday, serve as the terminus for trains arriving underground from Silicon Valley and points south.

Sunday, however, fewer than a dozen buses per hour were pulling into the third-floor bus deck that includes a direct ramp from the Bay Bridge. The new station stretches nearly three blocks between Second and Beale streets just south of Mission Street, and is wrapped in undulating curves of perforated white aluminum.

Now in business The rooftop park at Salesforce Transit Center is open from 6 a.m. to 9 p.m. seven days a week. Bus service to and from the transit center is provided by AC Transit, WestCAT, Muni and Greyhound, with links to SamTrans and Golden Gate Transit.

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The first bus from Oakland arrived shortly before 6 a.m., packed with transit aficionados and greeted by confetti, coffee and pastries. But by 8 a.m. the mood was businesslike — and most riders coming off buses were more focused on getting to work than pausing to savor the surroundings.

“I didn’t pay much attention. What bothers me is that the new station is inconvenient for the (Muni) buses,” said Aaron Carter, who stepped off an AC Transit bus on his way downstairs to catch Muni and head west to his job as a janitor.

In the new station, Muni buses traveling surface streets turn right from a protected boarding area onto Fremont Street — an artery often gridlocked in the morning commute.

Because of that, Carter said, “Nah, I’m not impressed.”

More enthusiastic was Derek Tillotson, a Treasure Island resident who walks to work from the transit center: “It looks different, and it looks nice because it’s brand-new,” he said. “I’ve been walking past this for years, and it’s been interesting to see it slowly come up.”

Construction began in 2011, after demolition of the Transbay Terminal that had occupied the then-remote site since 1938.

Now it’s the south edge of the Financial District, flanked by towers. Three are occupied by Salesforce, the tech firm that will pay at least $110 million over the next 25 years to help fund the center’s operations, maintenance and security.

The sponsorship agreement is the reason the publicly funded structure’s official name is Salesforce Transit Center.

For bus patrons with time to look around, the new surroundings met their approval.

“It reminds me of modern rail stations in Berlin and Europe,” said John Grunstad, who lives in Emeryville and helps manage safety boats for the Dolphin Club. “The open (aluminum) siding is nice, and the space doesn’t feel crowded or cramped or gloomy.”

After Grunstad checked out the rooftop park, he was even more impressed.

“This makes me very happy,” he said, smiling. “If you’re going to spend a lot of (public) money, do it for something like this that people will enjoy.”

The test for the bus deck comes Monday.

Only three AC Transit trans-bay bus lines run on the weekend, instead of 27. And those lines operate only once or twice an hour — rather than three or four times, as can be the case during peak workday commutes.

The bus deck also is the final stop for three Muni buses from Treasure Island per hour on weekends, compared with as many as six each hour on weekdays.

But even the less-frequent bus service was enough to keep Kahn’s fountain in action and park patrons entertained.

Laura Graffman, who comes from Oakland each weekend to join friends in the Bay Area Association of Disabled Sailors, visited the park on Sunday before heading to Pier 40.

“This really is something,” Graffman said. “I’m not a big fan of these big buildings around it. But if that’s the trade-off, I’ll take it.”

John King is a San Francisco Chronicle staff writer. Email: jking@sfchronicle.com Twitter: @johnkingsfchron