Obviously, if you already own these albums, don't buy this set. However if you don't own these albums don't even think twice, get them! Opeth stand as one of the best metal bands of all time and their first three albums (Orchid, Morningrise, My Arms Your Hearse) gathered in this set offers a fascinating look at their early years. They pretty much are becoming metal classics.
Orchid (9/10) is one of the most explosive metal debut albums of all time, as the group sound so fully formed and their style so recognisable right from the start. I love this album, there's just something about the production and the songs that just really clicks with me. Morningrise (8/10) is a great follow up record, with some truly haunting songs (Advent, The Night & The Silent Water) and more mellow acoustic sections than the first album. It doesn't quite match the debut album for me, but still has its own charms and intricacies. And finally, My Arms Your Hearse (10/10), a concept album, is excellent. Arguably, here Opeth completely find themselves, with less acoustic sections, but a wider variety of genre types (including jazz fusion!) this one is a really heavy harrowing album. It also includes concert staple Demon of The Fall!
So if you like prog and metal, and want to try something a bit darker than other prog-metal bands (like Dream Theater), Opeth are probably your best bet. If you're not a fan of very long songs (average Opeth song is about 8-10 minutes, some longer on these early albums) and sparse instrumentation, or growling vocals (though I used to hate it, but Opeth changed my mind about them) then probably best stay away - everyone else? Buy this set!