The first thing I must say is that if the version of Arena your looking at isn't at least V1.6, DO NOT buy it. There are some very serious bugs in versions up to 1.6 that will prevent you from finishing the game. In my case I had to find a certain temple to receive instructions about the next stage of the quest. The temple simply did not exist! Arena is a buggy game, crashes and lock ups are common. That being said, I will be the first to line up for Daggerfall. I saw enough potential in Arena to know how good it can be. Despite the bugs I really enjoyed Arena.
The basic plot of Arena is a rather standard fair. Find a weapon that is scattered in several pieces in various locations. Use the weapon in the final battle. Simple. You can follow the quest for the weapon parts quite easilly and the formula is fairly much the same for each part. Follow up rumours - do subquest to gain trust of those with knowledge of next piece - go on search for map that will show location of next piece - find next piece - on to next piece.
Combat is real time, but special magical weapons make battles much more easier. Magic is reasonable, you can cast spells during combat without too much formality. You can also create your own spells, although I did not persist with this idea for too long.
The sound affects are very good and the graphics are also excellent. I would not say the graphics are up to Underworld II standard, but Arena is a far richer and varied world. The digital sound effects of some of the monsters is the best I have heard. The main quest dungeons are varied, complex and well constructed. The sub quest dungeons are rather poor affairs that will become boring after several. The towns are huge and contain the usual weapon and armour shops and hotels.
The world of Arena is huge, there a scores of dungeons, towns, cities and mines. You can actually travel freely around this virtual world. Unfortunately there is little reward to venture outside the bounds of the quest itself. The game could have been improved out of sight had some rewards have been hidden in the countryside, like magical weapons or items. I don't know what went wrong, but theres a virtual world full of towns, villages, people and dungeons, but outside the bounds of the quest there is little point in exploring it. A real missed opportunity for the game designers.
All in all Arena is a good game, but be prepared to put in many weeks to solve it and tolerate the bugs.