Whether or not the thumbhole, is a good idea depends upon how you'll use the rifle, assuming that the one you get fits you. What I consider a plus is that the thumhole positions your hand so that you pull the trigger straight back, whereas most conventional stocks have your finger slightly above the trigger, so you're pulling up on it. This is a minor advantage though, as the majority of shooters with properly tuned triggers wouldn't be able to tell the difference. If the thumbhole has a major advantage its that the rifle takes on a straight line from barrel to butt much like a tube gun; this makes recoil more manageable, and would certainly be a bonus on a target rifle chambered for a powerful cartridge, like a .300 Winchester or a .338 Lapua.
But what of the disadvantages? If the thumbhole fits your hand well in warm weather, what happens when you go hunting late in the season? Will your thumb still enter the hole when you have gloves on? If its a struggle, how much time does it take to cycle the action and get back on target? Does the design allow you to shoot the rifle in a convenient manner with your thumb alongside the stock?
The deal breaker for me is the inability to conveniently shoot a thumbhole stocked rifle from my support side. Ambidextrous shooting while hunting doesn't come up that often, but it can be beneficial in some circumstances, and if you can do it, you're more likely to use it. Its easier to track an animal running right to left with the rifle on your right shoulder, but its easier from your left shoulder if the game runs from left to right. I had the occasion to use the technique on my warthog, which took off quartering to the right. At the shot I rolled the M-70 a quarter turn to the left, ran the bolt with my left hand, rolled it upright,and was immediately ready for a follow-up shot, but none was required. The reaction from my PH suggests that this is not a common technique, but the point is, that shot could not have been made with a right handed thumbhole stock. Now in the Canadian context, consider hunting from a canoe along a narrow river, where the moose appears on the right side of the canoe. What is quicker, spinning the canoe around for a right hand shot, or just shooting left handed? There is why I wouldn't want a thumbhole stocked hunting rifle.