Loan issue aside, I would advise almost everyone who isn't already "fuck you money" rich to not get too caught up on that extra 19% tax on income vs capital gains and just delay on exercising.
There are occasionally exceptions to this (involving a rare combination of the employee leaving and the company being as close to a "sure thing" as possible), but in the general case the risk/reward is so extremely out of whack that I can't imagine people would even consider exercising prior to the options becoming liquid if it weren't for the irrationally that comes into play when the money is going to the IRS.
There are just so many ways in which those options can become worthless (and a lot of them can happen even if the company is 'successful'): company goes out of business, company is acquired under terms unfavorable to common-stock holders, company dilutes stock to the point where your strike price is a joke, company actually IPOs but stock goes south of strike price due to massive selling pressure as lockup ends, etc, etc.
