All told, Betfair has sold off about 16m shares, equivalent to 15% of its stock (allowing current investors to cash out to the tune of over £200m). They were eventually priced at £13, at the upper end of the proposed £11-£14 range, and at time of writing, they've jumped a further 18% to £15.33 (having gone as high as £15.65). So it's safe to say that investors like the stock - even if that £1.4bn valuation (or £1.6bn as it is now) feels a teensy bit rich for a company that only had sales of £340m last financial year, and whose US prospects remain distinctly uncertain.
On the other hand, having gone pretty much from strength to strength during its ten-year history, there's every reason for investors to trust in the management and the software behind the site that has revolutionised online gambling. And since this valuation will presumably propel it into the FTSE 250, it should gain some extra kudos and profile as it looks to break into more of those all-important overseas markets. It's also a profitable, cash-rich and largely debt-free business, so it's expanding from a strong position. You can see why investors think it's worth a punt.