This is a long haul through the woods but the views up top are worth it, I promise! Look on the bright side and be grateful for the shade the whole way up. Once you get up, be sure to pan around to find the Green Mountain look out. 

Trail is not rocky which is nice, but is littered with blowdowns. The path to avoid them is obvious in most places, but it is good to pay attention on your way up so you remember how to navigate them on your way down. 

No water after the creek in the first mile (as of this writing). Fill up there if you need to. 

This trail is a good choice for those looking for a 4,000+ foot challenge without the crowds of the I-90 and Mountain Loop Highway clusters.  I hiked it on a high-summer Saturday in perfect weather and didn't encounter another soul.  The views from the top are spectacular, although frankly I think Green Mountain offers a slightly better perspective on the North Cascades with the added attraction of the lookout itself.  That is a popular trail, though, so if solitude is your thing and your quads are up to it, you might want to keep driving to Sulfur Mountain.

The Suiattle Road is in generally good condition following its 2013 repair, with some patches of potholes and slumps.  The roughest area is right after the pavement ends.  The parking lot was nearly full when I rolled in around 11 am, so next time I'd try to start a little earlier.

The trail description says it all: be prepared for miles of switchbacking through forest with only occasional glimpses of the peaks across the Suiattle valley.  The grade is brutally reminiscent of Mailbox Peak at first, but it settles down to a fairly steady but manageable ~15% incline as you ascend until a few steeper segments near the top.  The trees start to thin over the last mile, but you still don't get much in the way of views until the summit itself.  On the positive side, the trees offered plenty of shade on a warm day.  As mentioned, there is no water after a small stream uselessly close to the trailhead, so bring plenty of fluids.

While the having the trail all to oneself is refreshing, there are some downsides.  The tread is generally narrow and sometimes slightly overgrown, especially over the first mile or so, and it's easy to continue past switchbacks if you're not paying attention.  More significantly, at least a dozen trees are down across the trail.  Most are a foot or two in diameter and not difficult to cross, but a few require detouring around or clambering over, and one monster just a few hundred feet up the trail has to be climbed via branch stubs and the root ball.  With such little traffic to keep the trail clear, I also found myself blundering through dozens of cobwebs and sometimes used my trekking poles to try to ward them off.  If you don't like spiders, this may not be the best choice.

There are a few snow patches near the top, including one fairly long stretch just below the summit, but they're melting fast and didn't present route-finding problems.  The wildflowers at the top are lovely, perhaps just past their prime but still showing off.  Glacier Peak is impressively close to the southeast, and Green Mountain lives up to its name to the west.  The route description mentions trails down to Sulfur Mountain Lake, which glitters a deep shade of blue in its valley just to the north, but I didn't see any obvious routes and the descent (and ascent!) would be steep and involve a boulder field near the lake.

As I mentioned, the view-to-effort ratio of this hike might not be quite as good as other options like Green Mountain or Hidden Lake Lookout, but it receives a fraction of their traffic.  It's also great for conditioning; my GPS logged about 4,600 total vertical feet over 10.5 miles.  You can jog down most of the trail if that's your thing, although it was too steep in a few places and the fallen logs slow progress.  Whatever you do, bring lots of water!

If you want solitude hike Sulphur Mountain - starts at end of the Suiattle River Road. Started at 8 AM - got to summit ridge in less than 4 hours. About 4 big blow downs that needed gymnastic skills to get over (under), but most blow downs were step overs. A few spots, new route has been started around blow downs. Rest of trail is in excellent condition. Only a few bugs noticed while hiking, but the summit ridge had swarms of mosquitoes. Clothing, including head net, seemed to work the best for protection. Deet slowed the mosquitoes down for a while. Trail is in the trees until the top and then great views especially of Glacier Peak. On our last 4 trips to Sulphur, we have seen no one.

As the previous review stated, overall the trail is in good shape except there was ~40 down trees across the trail. Most of the trees were not hard to go over or under, except there was one big one that I couldn’t get my legs over, so I walked up and around it, as others had also done. The Suite River Road (FR 26) is paved for first 10 miles and last 12.5 miles are gravel, but overall in good shape, not a lot of pot holes that require you to drive super slow. The trail starts ~1/8 mile (on left) past the end of FR 26. Until you get close to the top (huffing and puffing) there are no views. Except for an almost dried up creek on the first half of the trail, there was not water. There was no snow. Unfortunately, it was partially cloudy, so I didn’t get to take full advantage of what I expect is normally an amazing view on a clear day. The majority of the hike is through a forest, so you don’t get any views until the top. There were some blooming wildflowers towards top. Bugs didn’t seem bad while I was hiking up, but once I was sitting at toward the top, the bugs were extremely annoying and wouldn’t leave me alone. While on the Sulphur Trail, I only saw one other group composed of three guys who were going on a weeklong backpacking trip.

The trail is in remarkable good shape given the fact the Suite River Road was closed from 2003 to 2014 (Matthew Riggen, Mt Baker-Snoqualmie National Forest, Darrington Ranger Station). For these 11 years, Sulphur Mountain trail #794 was virtually shut off from use and certainly maintenance. The trail starts 100 yards past the end of Forest Road 26, a road that is now as good as any backcountry road in the state. The first 300 vertical feet are some of the steepest tread in the state. At that point you arrive at the first and largest of the 57+ blowdowns and the trail assumes a pretty even and steep grade the rest of the way. There is a lot of duff and minor branches along the way and no views or water until assuming the ridge at 5 miles and approximately 4,500 feet of elevation gain. There is a little snow left along the route the last 400 vertical feet but the trail is easily traced.