If you are competing for the National Championships and are a top level sailor, some aspects of the Tiger design are a little bit dated and you'd probably notice the difference in performance. However, if thats not the case, then the boat is a fantastic value and the biggest differentiator with other boats would be the nut on the tiller.
I sail both a Tiger and a C2 on a regular basis and am always pleasantly surprised by how well the old girl sails. It seems to me there are conditions where the Tiger may have advantages over the newer boats (especially short chop, lighter winds). There are guys sailing Tigers in our fleet that do quite well against the newer boats.
Overall, its an excellent, well-proven boat, and would be a fantastic choice as your first F18. Once you get your skill level up, then you can sell it for a large percent of your initial investment (they don't depreciate as quickly as the newer boats) and then upgrade to the latest/greatest.
'88 H18 Jolly Mon
'10 F18 Closely Calledcramsailing.com