wow, I can't keep up with all the new zill releases. Good thing they are releasing the orange sherbet. I have an OS tree (err actually 2 of them) that have produced fruit for the last 3 years.
I selected 2 of the most promising seedlings from a batch of fruit purchased from Sir Walter (based on leaf and smell characteristics) and grafted them to a cut back and vigorously growing, mature mango tree (I lifted a page from David Sturrock's playbook on that one). Within 3 years, the trees started fruiting. But I had been waiting for Zill's to release before publicizing it, as I didn't want to infringe on Gary Zill's exclusivity. (I'm still not releasing budwood until the cultivar has been officially released.)
Both of my trees are absolutely identical to the parent, in both tree and fruit. After several years of eating the fruit and about 5-6 years of managing the tree, I'd say it's my least favorite of the Lemon-X club (consisting of Lemon Meringue, Lemon Zest, and Orange Sherbet). The tree is ridiculously vigorous. In fact, I think it puts Valencia Pride to shame. It does seem to set fruit from flower rather well. However, it grows so egregiously rapidly that my annual prunings (we're talking 15+ feet height reduction each year) preclude 2/3 of the tree from flowering.
As far as fruit quality goes, it does get jelly seed fairly easily. The flavor is good, but it has a more intense "lemon" component to it than the LZ and LM -- can't really describe this.
I know Alex has mentioned that LZ has susceptibility to black spot, and perhaps that is the one area in which OS excels? It's possible that my experiences with the tree are aberrant, but they seem to coincide with Walter's. I do think it's worth growing, but an LZ or LM may be preferable if space is lacking.