In my opinion: (BTW, I've been lucky enough to drive all 3)
The Boxster wreaks of "I wanted a 011 but couldn't afford it". However, it is a ncie car with great handling. looks nice too. Only problem is that it's not too quick. Not as fast as I thought it would be anyway.
Elise is a ton of fun. But not without it's problems, it's tiny and getting in and outmakes you look like a fool. Personally, I like the styling. It's handlin is incredible, you turn the wheel and it responds immediately. It's a great car, but you definately won't want to drive it everday. Obviously, the Exige is everything I've said multiplied by ten. I absolutely love that car and if I could afford it, I'd have one. FWIW, even while just cruising at highway speeds, the interior on the Elise is a bit noisey.
The Merc SLK55. Very fast, it has a wall of torque that just doesn't seem to end. Untill you hit 155mph anyway and it doesn't take long to get there. It's everything the Boxster is, but a bit faster in acceleration. On a twisty road or track/autox, a Boxster will be all over you like a junkie on smack. I understand why people like Merc's, they look good and they are quick. Plus the ladies dig them, but they have a huge problem. The dealers treat youlik crap and they aren't of the best quality and like to break down. They are loaded with sensors and other technology that is nothing more than a headache. For those reasons alone, I would not recommend a Merc to anyone.
Out of the three, I'd recommend the Boxster. I'd also give the Cayman a look if it's within your price range. You could easily use a Boxster as a daily driver as well. There is a downside to Porsche ownership though, people will give you the finger a lot. As well as act like pricks on the road. To most people, Porsche=Rich. I've driven my boss's 996TT around numerous times and it's unbelievable how people will treat you based on what you are driving.
If I can, I'd like to toss another car into the mix, the Audi S4. Fantastic car in every respect IMO. The resale values are horrid though. They devalue very quickly.