Ignoring temporary inflation due to businesses taking advantage to round things up in Euros, my belief is the switch will be a net positive.
Croatia has been already completely tethered to Euro. Kuna was unofficially pegged almost from the start, house and car prices are always quoted in Euros, people's mortgages are calculated in Euros. The "loss" of monetary freedom that comes with not being able to print your own currency is not a loss here because Croatia could in practice never leave the peg, the consequences would be extremely severe.
On the other hand, Croatia is a very small country (less than 4m people) and tourism makes up a large portion of the economy. Using Euro removes friction and costs tourists and businesses had having to convert from/to the currency. The removal of the border checks (entering the Schengen, which also happens on Jan 1st) will also help with this.
> other countries like Finland regretting the switch from their currency as they don't want to have to suffer sky high inflation and be punished for being financially responsible just to help other countries like Greece or Italy pay their huge debt.
Croatia will be a net beneficiary (edited, thx) of EU funds related to the integration processes for at least a couple of years, and is near the rear of the pack in most economic measures.