perseus' downloadable content isn't what makes it so useful--you can just buy the texts from bookstores. it's the searches you can't do anywhere else. in my favourites bar i have a dynamic cluster search of homer: type in any word and you can see what other words most naturally/frequently go with it. i have my sophocles word in context search to find words used in iambic tri so that i can find their correct scansions, and my "other playwrights" word in context search in case the word isn't in sophocles. using this you can scan prose, since you don't have to worry about iambic shortening like in latin. i have my lsj search which i use for almost every tricky word while working through north & hillard, greek prose composition: perseus tells you instantly whether you've guessed a legitimate word or not. you can't do any of this outside perseus, and so it's a necessary resource for classics beginners who don't have a teacher to correct them.
the only thing that annoys me about perseus is that you can't search for a specific inflection of a word in the word in context searches. e.g. if you search for the dative singular of a noun, even if you check "exact matches only", it gives you all cases and numbers of the noun; so if you have a really common noun which usually comes up in the nominative (e.g. the word for odysseus), you have to wade thru pages and pages of hits to find your dative singular.