A Microsoft product without different versions would be like Holmes without Watson, and Office 2008 is no different as you can see below:
The eagle eyed among you will spot that there are basically two differences between the £90-odd Home and Student edition and the £300-odd standard edition of Office 2008. First, there is the huge price gap and second there is the addition of Exchange server support and Automator Actions to the more expensive version.
The third version also packs in Microsoft Expression Media, which is a multimedia management package similar to programs like Photology and you'll need to decide for yourself whether it's worth your hard earned cash.
Whether the improvements in Office 2008 for Mac are significant or not, they are all improvements and as such it's well worth the investment if it addresses your needs. However, if you're looking to purchase Office then the recommendation comes with a small proviso. If you don't need Exchange support then I would unreservedly recommend that you spend the very reasonable £90-odd required for the Home and Student version.
If you do need the extra features offered by the full fat Office version, the extra £210 premium might be a bit harder to swallow for very little extra in the way of features - that's the price of using a Microsoft mail server. As anyone using Exchange will most likely be doing it on a corporate or maybe educational network, and thus probably not buying office themselves, the recommendation is not to buy it so much as to persuade whoever has control of your budget to approve the purchase. As long as you don't need VBA support, which is a potentially big if, you won't be disappointed.
Whatever your situation, if you do get hold of a copy of Office 2008, you'll definitely be pleased. It's the cleanest, fastest, most user friendly version of Office thus far produced and for that reason it gets a wholehearted recommendation.