It's always a gamble certainly, but rarely a gamble that pays off; I'm not saying they can't result in a higher overall fee, but they rarely do. There is always a take off before an add on.
If he goes to SPurs or Arsenal and is great, very few are realistically able to come in and take him away from there; that pool of clubs is very small. I'd be keener on a sell on clause if he went to Forest as there is a realistic chance of his moving onward and upwards from there. If he leaves Arsenal or Spurs, it's likely to be in a downwards direction for less money, making a sell on clause useless.
If a club is willing to pay X + a sell on clause, they were willing to pay X + more up front too. If you want a sell on clause, you have to accept less up front. People feel very clever when sell on clauses pay out, but I'd guess in 9/10 cases they don't.