I have top tier with three airlines (and therefore 3 alliances) Delta (Plat), BA (Gold) and BD (Gold) (Skyteam, One World and Star Alliance).

Until quite recently, I could have sung the praises of DL Platinum. It was far superior to Gold and Silver in many ways, not least of all upgrades, lounge access, and the general treatment that I got from DL employees. They really went the extra mile for Plats, somewhat more than they did for Gold and quite a lot more than for Silver. Upgrades were great, and it was truly a superior program. Unfortunately with the SWU changes that have been announced as well as some attitude (not to mention personnel changes at DL) this status seems to quickly be losing its lustre. As for Skyteam, although it is new, my only experiences have been with AF whose employees couldn't seem to care less about your status (i.e. I've been refused access to Af's lounges, even when showing written proof that I coulkd use them, and have had loud arguments with "line ushers" at CDG who insist that I must get out of the Business/First line when checking in).

BA's program is poor. As a silver you could use lounges, but they didn't care much about me. As a Gold, they don't seem to care anymore than they did when I was silver. I will say however that Iberia and Cathay seem to treat me better with my BA credentials than BA itself.

BD, really treats both its Silvers and Golds quite well. They seem respectful, and most importantly, seem to nactually bump people off flights to get you on. Supposedly they are good with upgrades on a Space available basis, but I haven't had the chance to experience it yet. As for Star Alliance, apart from a few LH flighs I have yet to test how respected their credentials are with other carriers.