The Frisian language is roughly as similar to English as Spanish is to Italian. I would say that it is somewhere between Dutch and English.
There is a famous little rhyme, which goes:
Good butter and good cheese
is good English and good Fries.
Though it is said that the Frisian version is:
Good butter en green tzieze (cheese)
is good English en good Friese.
This rhyme overstates the similarity. If immersed in the language as a native speaker of English, you could probably pick it up relatively quickly. But it is still a foreign language:
Us heit, dy't yn de himelen binne; jins namme wurde hillige. Jins keninkryk komme. Jins wollen barre allyk yn 'e himel, sa ek op ierde. Jow ús hjoed ús deistich brea. En forjow ús ús skulden, allyk ek wy forjowe ús skuldners. En lied ús net yn forsiking, mar forlos ús fan 'e kweade, Hwant Jowes is it keninkryk en de krêft en de hearlikheit oant yn ivichheit..
Did you get that? It's the Lord's Prayer in Frisian. I rather think that the Romance Languages are easier to grok. Damn Normans.
As far as grammar goes, it differs from English most in that its nouns have two genders. Like English, the only cases they use are genitive- which is used less frequently than in English, keeping up the trend of Germanic languages becoming more and more isolating, like the Chinese dialects. Apart from that, everything is different, but in a more subtle way.