The 34-year-old Finn has been rewarded with a testimonial this summer, having made 425 appearances since his £100,000 transfer from VPS Vaasa in November 1997.
"I think most people recognise the qualities Jussi has. He is certainly not undervalued at this place," said Megson.
"He has proved to be a fantastic signing. Jussi came to us as raw material.
"I think we have the best goalkeeping coach in the Premier League in Fred Barber and he has done a great deal of work with Jussi and he has got better and better.
"He's like a fine wine, as he gets older he gets better and we wouldn't swap him for anyone else."