Firstly I must start buy saying that if you are thinking of buying 'Rome Total War Gold Edition' then buy this instead. It is the same price and you get the 'Alexander' expansion, bonus material, and two other games with expansions.
I already own 'Shogun Total War' and 'Medieval Total War', and so decided to complete my collection. Just as I was preparing to buy 'Rome Total War', I discovered 'Total War Eras'.
This pack is GREAT value for money.
'Shogun Total War' is the first 'Total War' game. Although the graphics are pretty bad, it still fun to watch your deadly ninjas stalk an unsuspecting general. This game is really fun for a time when you just want a quick, easy game to play. 'Mongol Invasion' is not a great expansion, but nonetheless is good value. It lets you play as the invading mongol hordes and, though hard, is fulfilling.
'Medieval Total War' is the next step up from 'Shogun'. Its brings in a bit better graphics (though still pretty bad) and a much more detailed type of gameplay. The map spans from Europe to Egypt, and the battles are much upgraded. 'Viking Invasion' is a much better expansion than its 'Shogun' counterpart. It gives you a whole new detailed map, with new nations and units ready at your command.
'Rome Total War' is the final step. It has excellent graphics and a revolutionized campaign system. New factions, buildings and units create an amazing gaming experience. Both expansions carry on the great game, introducing excellent new things.
All the bonus equipment is...well...a bonus! Its quite fun, but nothing amazing.
For new 'Total War' players - buy this! It's a great way to get completely stuck into the games.
For old hands - If your missing any of the games, or just want to relive the fun, buy this! If you have them all, then maybe its best to wait for 'Medieval Two'.