The deals you see for £99 are almost always to San Antonio and will be in unnamed accomodation, so you won't find out where you are staying until you arrive. This might be fine for you, however I'd personally find it too stressful worrying about whether I'd end up in a dump.
Also, a lot of the time, there are conditions involved. For example £99 might be per person based on about 4 sharing. So if there's only two of you, you might get lumbered with a hefty under occupancy charge.
These sort of offers tend to be too good to be true, however if you are able to go to a travel agent and say that you are prepared to go in about a couple of days time, you will probably get some good deals. Although unlike other resorts, the price for Ibiza really doesn't seem to drop the nearer you get.