Life was mostly uncomfortable—at least by modern traveling standards. Our information about sailing longships comes mostly from two batches of modern replicas: one set built on early Viking-Age boats from Norwegian burials, and another set built from late Viking-Age boats that were scuttled to form an underwater barrier in Denmark. I'm personally most familiar with Skuldelev 2 from Denmark, and it's modern recreation the Sea Stallion. It should be noted that none of the archaeological finds included vital parts like sails and rigging, so many of the technical conclusions drawn from these replicas are actually best guesses based on what works.
To look more closely at Skuldelev 2, the original ship was about 30m (100ft) long with a maximum width of 3.8m (12ft). It looks like she had spots for about 60 oars, so probably a crew of 65-70. She's only 1m (3ft) in draught, so not very deep at all. The 'deck' is really just a surface of boards that cover up the awkward parts of the ribs to make the ship a bit safer for people on board. On the modern replica, many of these can be removed to access individual storage spaces, which can fit perhaps the contents of a bookshelf or two, although at the middle of the ship below the mast, many are filled with stones for ballast to keep the ship from tipping over.
Overall, things are pretty cramped—especially if the ship were packed for a multi-day voyage across the North Sea. When the crew is rowing into a harbor, they don't really have room for full arm extensions, and it requires a lot of discipline to make sure the oars don't knock into each other (or unwary crewmembers). There is enough space for people to squeeze past each other, and on longer voyages the off-duty crew will often sit back or sleep between the 'benches' while on-duty crew use the benches as they handle the sails and trim the boat. In rough seas or sailing against the wind, everyone is involved in trimming the boat, or making sure that she stays balanced and upright. It's a bit like a bicycle: if it stops, it falls over. The off-duty crew is always being used as mobile ballast, shifting from side to side to make sure the sail doesn't blow into the sea, scoop water, and capsize the boat. A skilled skipper can actually use the bodies of the crew to change or maintain direction without dressing the sail, since the force of the wind driving the mast into the keel can be redirected by shifting how the boat is balanced around that central spot.
There was also a hefty bit of discipline and hierarchy. The skipper at the stern has no visibility of the foreship, and in wind or wave it's almost impossible to hear a human voice at that distance. Typically an experienced crewmember stands at the mast, relaying messages while controlling that human ballast at the middle. In the stern, the skipper commands the rudder as well as the ropes tied to the top and bottom of the sail (the yard and the sheet). In the foreship, an experienced crewmember manages the ropes which control the middle and front of the sail (the tack), and that crewmember is also responsible for making important navigational calls. On a ship the size of Skuldelev 2/the Sea Stallion, controlling the ropes is a group effort, since the sail is made from over 100 sq m (1000 sq ft) of heavy water-proofed wool. The foreship is especially unforgiving, with spray coming over the bow, the sail to the rear providing no shelter from the sun, and exposure to the wind driving temperatures several degrees lower than at the stern. The modern skippers typically places their younger and more junior crewmembers up front.
As you can tell, this is all pretty thankless. Doing it a few times has a bit of adventure to it, and becoming seasoned at it can gain a crewmember responsibilities that distract from the hardships as well as privileges that ameliorate them. But overall, every good skipper knows that the best bet is to stay close to land, put in at night whenever possible, and avoid sailing in rough or rainy weather whenever possible. By the late Viking Age when ships like Skuldelev 2 ruled the waves, viking fleets were typically pretty substantial. I suspect that deeper cargo ships called knorrs probably transported much of the necessary supplies and maybe even some members of the war party. But overall, you pretty much just had to grin and bear it if you were a lower-ranked sailor/raider, probably with just a single set of wool clothing and perhaps a wool blanket waterproofed with animal fat.
Early Viking Age ships, in my opinion, might actually have been somewhat more comfortable. They tended to be wider and deeper, and recent work on the Gokstad ship indicates that rough weather actually caused the ship to ride higher on the waves, keeping the crew dry. These ships weren't dedicated warships (which actually means personnel carriers), so there was ample room for a limited amount of cargo both on and below decks. It seems like each crewmember had his (or possible her) own sea chest, and that the crew would use these chests as benches for rowing. But unlike in the later Viking Age, early ships travelled only in small groups, so all necessaries needed to be kept on board. Overall, I suspect that sailing on a ship like the Gokstad ship would have been more comfortable for the 35 or so crewmembers on board, but from the perspective of a war leader, a faster warship that could carry more bodies may have seemed more useful.
The Sea Stallion has perhaps the best documented voyages online, and crews have sailed her from Denmark via Norway to Dublin, then back via the English Channel to Denmark. A quick search should turn up more than enough viewing material to keep you interested, and the Viking Ship Museum has some online materials on the replica voyages.