I visited Taxila from Islamabad a few days ago. I would recommend the trip to anyone who is spending more than a few days in Islamabad, but I did much prefer Makli and the Mughal sites in Lahore (I haven't visited Rohtas Fort, Moenjodaro, or Takht-i-Bahi). The positives of visiting Taxila are the ease of getting here, the large number of sites, and how unique it is to see ancient Buddhist artifacts in Pakistan. The negatives were the visible degradation, the lack of helpful signage to understand what you are seeing, and the too persistent guides.

Positives:

The museum, not technically one of the UNESCO sites, is where you should start and is one of the nicer museums I've been to in Pakistan. Taxila is unique for the multitude of different cultures, peoples, and religions that coexisted and succeeded each other. You will hear about, if not really understand that you're seeing evidence of, Greek paganism, Jains, Zoroastrians, and Christians, in addition to the Greco-Buddhist art. If you have visited the temple of the tooth in Kandy, you will be pleased to see that the Thai government has sponsored an exhibit here where two other of Buddha's teeth supposedly are. I found the Greek artistic influences on the Buddhas and scenes of worshippers fascinating. You can see evidence of this influence in the Corinthian columns, hair swirls, and the style of the togas. Some of the scenes in relief are preserved in great detail, but it would have been great if the placards explained them.

After the museum, I visited Sirkap, then Dharmarajika, and briefly Jaulian. The signage for driving is great. I hired a car for the day and it is best to follow the signs as the Google maps location for Dharmarajika is off. 

Negatives:

I'll preface this by saying I did not hire a local guide, just a car for the day. At other sites in South Asia, I've found guides not super helpful as well as pushy. I also like to set my own pace (I only spent about 3 hours across all the sites). Because of this, YMMV.

The artifacts in the museum have sadly been stripped away from where they were found. When walking through the museum, it is a bit unclear which artifacts belonged to which sites. Placards mostly just note the subject/title and approximate year. Despite having done some background reading in advance on the history of successive civilizations, I still found it hard to follow. When walking around the sites outdoors, one wonders what the placements of these relics might have been.

In addition, farmers and shepherds have access to the excavated and un-excavated sites and will be walking with their animals by you. When they plow the land, apparently they find new relics. This was told to try to convince me to buy some coins supposedly found here, which I refused. Either they are real and should not be looted from the sites, or (most likely) they are expensive forgeries. Still, I don't doubt that locals do come across artifacts, as the sites are left fully unprotected and only about 10% of some places like Sirkap city have been excavated. Taxila as a whole, despite being much cleaner than Makli, for instance, seems more at active risk of losing its history.

Finally, the guides. From the museum onwards, I could not shake off guides. This was a theme of my whole visit, perhaps because I was a solo American woman and it was relatively empty during Ramzan, and it made me cut my trip short. This was in spite of multiple precautions I took. First, I traveled by a normal unmarked car with no fanfare. Also, I wore an abaya, dupatta, and mask to try to avoid any attention. I would recommend similar conservative clothing and not attracting attention to any foreigners. It started with the first guide at the museum, who was nice but not very helpful. In the end, I had to tip 3 guides within the museum, because they kept wanting to show me something else, and one at each of the sites I went to, in addition to paying for parking. I could have been more persistent in asking to be left alone, but the male guides would relentlessly follow me. In one case, a man literally ran after me. Since there was almost no signage and I was slightly worried about my safety, I did not press the matter. At one point at Dharmarajika, I heard some locals yelling in Urdu to a potential guide to come speak English to the white girl. I've solo traveled around South Asia off and on for the last 7 years, was speaking to them in Urdu, and wearing culturally appropriate dress, and still felt like this was a next level of annoyance.

I'm not sure what I would recommend doing differently to be able to take full advantage of a world heritage site, but left frustrated after just a few sites. I hope this insight can be useful to someone else visiting.