To sum it up right at the beginning: There is not much to see at the Danube Limes. Most of the remains are hidden under fields and meadows or have been overbuilt by later settlements and newly founded towns. There are hardly any larger visible structures. Often only some foundation walls are visible or the original remains are preserved as components of later buildings.
On several trips in the last years, since the Danube Limes appeared on the tentative list in 2015, I have visited most of the locations with visible remains. The nomination files provide detailed descriptions of each site and what you can see there. For those who don't want to go through more than a thousand pages, I will summarise my visits in this review. I will focus on the most rewarding locations and those that can easily be combined with other visits because they are within the core zone of other World Heritage Sites (Regensburg, Vienna, Wachau) or close to them.

The numbering of the multiple locations is confusing: the Unesco website lists 77 locations with consecutive IDs. In the nomination documents, locations within the same buffer zone are grouped together and listed as sublocations (a, b, c...). I specified both IDs in brackets (ID Unesco website/ID nom file).

Germany

The Danube Limes WHS starts with the remains of a thermal baths in Bad Gögging (1/1), about 50 kilometres southwest of Regensburg. Only a small part of the baths has been excavated and can be visited in the Roman Museum (located in a former church). You can combine it with a visit to the nearby Abusina fort, which is not part of the Danube Limes although it lies on the banks of the Danube. It is considered the last site of the Rhaetian Limes. I wonder what the Romans would have thought of this classification. Abusina is interesting as it is one of only a few completely excavated fortifications on the German Limes. The foundation walls have been reconstructed, so that the ground plan of the fort is visible. However, for a visit to Bad Gögging only, I would not recommend planning your itinerary according to the limited opening times of the thermal baths (weekends only).
The Porta Praetoria (7/6a) in Regensburg is certainly one of the easiest ways to tick off this WHS. It was the main gate of the Roman fortress and the original remains are clearly visible in the whitewashed façade of the later building. The site is mentioned in most guidebooks and I guess that many WH enthusiasts stood in front of it without suspecting that these few minutes would count for another WHS. If you follow the north-west and western boundaries of the historic old town, you will find more visible remains of the Roman legionary fortress (9-14/6c-h). Most of them are parts of later buildings. Clearly historical, but without the information boards you would not necessarily recognise them as Roman. Worth mentioning is location 12/6f (named document Legionslagermauer), a wall several metres high, uncovered during the construction of a multi-storey car park.

Austria

Austria has more to offer than Germany, with the best sites being in the eastern part of the Limes. In the west, only Burgus Oberranna (25/10) is worth mentioning. The remains are covered with a protective building, so pay attention to the opening hours, entrance is free. However, I would recommend a visit only if you are in the area anyway.
For those travelling by car, the section between the Wachau WHS and Vienna might be worthwhile. Free-standing parts of fortification walls and horseshoe-shaped towers can be found in Mautern (51/24a) and Zeiselmauer (61/28a und 64/28d). They have been preserved because they were incorporated into the medieval town walls. Although these remains are among the largest visible structures on the Danube Limes, it is not enough to get an idea of what an ancient Roman fort might have looked like. Traismauer is known for its Roman Gate (57/25e). But the name is misleading, in fact the building in its present appearance dates from the 17th century. The original walls from Roman times can only be seen in the lower part. In contrast, the horseshoe-shaped tower in Tulln (59/271) remained almost completely preserved up to the roof at a height of 10 metres.
Vienna is another easy tick, but the visible remains are even sparser than in Regensburg. Every visitor knows the Michaelerplatz (67/30b). In the middle of the square, a small archaeological site remained uncovered offering a glimpse of the city's history that has been preserved beneath the pavement. But you have to look closely and study the information board to identify the Roman walls next to medieval and more recent remains. There is a little more to see at Hoher Markt (High Market, 70/30e), but also below the street level. In the basement of the small Roman Museum are the remains of the tribune's houses.
And finally, Carnuntum (71/31), with 1400 hectares by far the largest of all the inscribed locations and certainly the best and most rewarding site to visit. Carnuntum was the administrative centre of the province of Pannonia. The site is interesting from an archaeological point of view because both the Roman civilian settlements and the legionary fortress have not been built over in the following centuries. The original remains comprise the ruins of a thermal bath, two amphitheatres, and the Heidentor (Heathen's Gate, photo). The archaeological park is completed by reconstructions of a city quarter with a public bath and several residential houses.

Slovakia

Of the two sites in Slovakia, I visited Gerulata, located south of Bratislava in the suburb of Rusovce. Component 72/32a comprises the foundation walls of the Roman fort, next to it an exhibition building of the Bratislava City Museum.

Hungary

At the WHC 2021, Hungary has withdrawn all its components at short notice. After long discussions the WHC decided to inscribe the remaining components in Germany, Austria and Slovakia. It is not yet clear how Hungary will proceed, if and how its locations will be included in the list. This is a pity, because Budapest in particular could contribute several sites with significant remains.

Taken together, this World Heritage Site certainly doesn't fill a visitor with enthusiasm. The visible remains are too sparse to get an idea of the structure, function and history of this frontier. Of course, visibility is not the only criterion. Modern archaeological methods such as aerial laser scanning or geophysical techniques are yielding amazing new insights. To learn more, you have to rely on museums. Along the Danube Limes there are numerous local museums, but in most cases the concept of the exhibition is outdated, boring presentations of the same finds over and over again. I have visited a few of them and in my opinion the museums in Vienna and Passau (Boiotro) are among the better ones. However, there is much room for improvement in the way this WHS is presented.